diff --git a/rfg_adc_plotter/gui/pyqtgraph_backend.py b/rfg_adc_plotter/gui/pyqtgraph_backend.py
index de6350a..8516dc8 100644
--- a/rfg_adc_plotter/gui/pyqtgraph_backend.py
+++ b/rfg_adc_plotter/gui/pyqtgraph_backend.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import math
 import signal
 import os
 import sys
@@ -31,7 +32,12 @@ from rfg_adc_plotter.processing.calibration import (
     save_complex_calibration,
     set_calibration_base_value,
 )
-from rfg_adc_plotter.processing.fft import compute_fft_complex_row, compute_fft_mag_row, fft_mag_to_db
+from rfg_adc_plotter.processing.fft import (
+    compute_distance_axis,
+    compute_fft_complex_row,
+    compute_fft_mag_row,
+    fft_mag_to_db,
+)
 from rfg_adc_plotter.processing.formatting import compute_auto_ylim, format_status_kv, parse_spec_clip
 from rfg_adc_plotter.processing.normalization import (
     normalize_by_complex_calibration,
@@ -49,6 +55,7 @@ from rfg_adc_plotter.types import SweepAuxCurves, SweepInfo, SweepPacket
 RAW_PLOT_MAX_POINTS = 2048
 RAW_WATERFALL_MAX_POINTS = 2048
 BSCAN_MAX_POINTS = 256
+BSCAN_LEVEL_TAU_S = 2.0
 UI_QUEUE_MAXSIZE = 128
 UI_MAX_PACKETS_PER_TICK = 8
 DEBUG_FRAME_LOG_EVERY = 10
@@ -1346,6 +1353,8 @@ def run_pyqtgraph(args) -> None:
     last_packet_processed_at: Optional[float] = None
     axis_range_cache: Dict[str, Tuple[float, ...]] = {}
     image_rect_cache: Dict[str, Tuple[float, ...]] = {}
+    bscan_axis_cache: Dict[Tuple, np.ndarray] = {}
+    bscan_levels_state: Dict[str, Optional[Tuple[float, float]]] = {"held": None, "held_bg": None}
     curve_has_data_cache: Dict[str, bool] = {}
     item_visibility_cache: Dict[str, bool] = {}
     text_value_cache: Dict[str, str] = {}
@@ -1441,6 +1450,43 @@ def run_pyqtgraph(args) -> None:
         image_rect_cache[key] = values
         return True
 
+    def stable_full_scale_distance_axis() -> Optional[np.ndarray]:
+        """Full-scale B-scan distance axis, computed once per (range, mode).
+
+        The per-sweep ``runtime.ring.distance_axis`` jitters when the reported
+        ``f_max`` varies between sweeps, which rescales the B-scan y-axis every
+        frame. Deriving the axis from the configured range keeps it stable and
+        only recomputes when the operator changes ``f min``/``f max`` or the mode.
+        """
+        key = (
+            round(float(runtime.range_min_ghz), 9),
+            round(float(runtime.range_max_ghz), 9),
+            str(fft_mode),
+        )
+        cached = bscan_axis_cache.get(key)
+        if cached is not None:
+            return cached
+        if display_distance_transform_enabled(fft_mode):
+            # positive_only_exact derives its step from real sweep geometry, so a
+            # synthetic 2-point freq array would be wrong. Hold the first valid
+            # real axis instead (still keyed by mode, so it re-seeds on switch).
+            real_axis = runtime.ring.distance_axis
+            if real_axis is None:
+                return None
+            axis = np.asarray(real_axis, dtype=np.float64).reshape(-1)
+            if axis.size <= 0 or not np.all(np.isfinite(axis)):
+                return None
+        else:
+            synth_freqs = np.asarray(
+                [runtime.range_min_ghz, runtime.range_max_ghz], dtype=np.float64
+            )
+            axis = compute_distance_axis(synth_freqs, fft_bins, mode=fft_mode)
+            if axis is None or axis.size <= 0 or not np.all(np.isfinite(axis)):
+                return None
+        bscan_axis_cache.clear()
+        bscan_axis_cache[key] = axis
+        return axis
+
     def set_item_visible_if_changed(key: str, item, visible: bool) -> bool:
         visible_value = bool(visible)
         if item_visibility_cache.get(key) == visible_value:
@@ -1887,6 +1933,10 @@ def run_pyqtgraph(args) -> None:
         runtime.background_buffer.reset()
         if clear_profile:
             runtime.background_profile = None
+        # Re-seed the held B-scan color levels so contrast doesn't drag an old
+        # scale across the background on/off transition.
+        bscan_levels_state["held"] = None
+        bscan_levels_state["held_bg"] = None
         runtime.current_fft_complex = None
         runtime.current_fft_mag = None
         runtime.current_fft_db = None
@@ -1905,6 +1955,9 @@ def run_pyqtgraph(args) -> None:
         runtime.ring.reset()
         axis_range_cache.clear()
         image_rect_cache.clear()
+        bscan_axis_cache.clear()
+        bscan_levels_state["held"] = None
+        bscan_levels_state["held_bg"] = None
         runtime.current_distances = None
         runtime.current_fft_complex = None
         runtime.current_fft_mag = None
@@ -2526,6 +2579,7 @@ def run_pyqtgraph(args) -> None:
         except Exception:
             fft_mode = "symmetric"
         runtime.ring.set_fft_mode(fft_mode)
+        bscan_axis_cache.clear()
         reset_background_state(clear_profile=True)
         runtime.current_distances = runtime.ring.distance_axis
         runtime.current_fft_mag = None
@@ -3765,7 +3819,16 @@ def run_pyqtgraph(args) -> None:
                 if display_distance_transform_enabled(fft_mode) and disp_fft_display_axis.size == disp_fft.shape[0]:
                     disp_fft = disp_fft[::-1, :]
                     disp_fft_display_axis = disp_fft_display_axis[::-1]
-                distance_bounds = resolve_axis_bounds(disp_fft_display_axis)
+                # Use a stable full-scale distance axis for the y-range/rect so the
+                # B-scan always spans ~0..12 m and never rescales per sweep. Fall
+                # back to the per-frame axis only until a stable axis is available.
+                stable_axis = stable_full_scale_distance_axis()
+                if stable_axis is not None:
+                    distance_bounds = resolve_axis_bounds(
+                        display_distance_axis_for_mode(stable_axis, fft_mode)
+                    )
+                else:
+                    distance_bounds = resolve_axis_bounds(disp_fft_display_axis)
                 if distance_bounds is not None:
                     d_min, d_max = distance_bounds
                     set_image_rect_if_changed("fft_waterfall_rect", img_fft, 0.0, d_min, float(max_sweeps), max(1e-9, d_max - d_min))
@@ -3776,6 +3839,28 @@ def run_pyqtgraph(args) -> None:
                         y_bounds=(d_min, d_max),
                         padding=0,
                     )
+                # Smooth the color levels with a slow EMA so brightness stops
+                # flickering while still tracking genuine signal-strength changes.
+                # Background-subtracted levels use a separate slot (different scale).
+                level_slot = "held_bg" if active_background is not None else "held"
+                if levels is not None:
+                    prev_levels = bscan_levels_state[level_slot]
+                    if prev_levels is None:
+                        smoothed_levels = (float(levels[0]), float(levels[1]))
+                    else:
+                        alpha = 1.0 - math.exp(
+                            -(interval_ms / 1000.0) / max(1e-6, BSCAN_LEVEL_TAU_S)
+                        )
+                        smoothed_levels = (
+                            prev_levels[0] + alpha * (float(levels[0]) - prev_levels[0]),
+                            prev_levels[1] + alpha * (float(levels[1]) - prev_levels[1]),
+                        )
+                    bscan_levels_state[level_slot] = smoothed_levels
+                    levels = smoothed_levels
+                else:
+                    # No fresh measurement this frame: reuse the held levels so the
+                    # image never flashes to autoLevels.
+                    levels = bscan_levels_state[level_slot]
                 if levels is not None:
                     img_fft.setImage(disp_fft, autoLevels=False, levels=levels)
                 else: