diff --git a/testbench/gui.py b/testbench/gui.py
index a63a8bc..5aa2c9a 100644
--- a/testbench/gui.py
+++ b/testbench/gui.py
@@ -724,6 +724,7 @@ class TestbenchGUI(tk.Tk):
         self._stop_log()
         if self.worker:
             self.worker.stop()
+            self.worker.join(timeout=5)
             self.worker = None
         if self.bench:
             try:
@@ -1130,9 +1131,11 @@ class TestbenchGUI(tk.Tk):
             "success",
         )
 
-        # Stop the normal worker so the sweep owns the instruments
+        # Stop the normal worker and wait for it to finish so the
+        # sweep thread has exclusive access to the instrument bus
         if self.worker:
             self.worker.stop()
+            self.worker.join(timeout=10)
             self.worker = None
 
         # Restart _poll so it drains the sweep data queue
@@ -1190,6 +1193,13 @@ class TestbenchGUI(tk.Tk):
         elif l_start > l_stop and l_step > 0:
             l_step = -l_step
 
+        # Clear any stale state
+        time.sleep(0.5)
+        try:
+            bench.supply.get_error()
+        except Exception:
+            pass
+
         # Sanity check
         max_v = max(abs(v_start), abs(v_stop))
         min_v = min(abs(v_start), abs(v_stop))
@@ -1312,6 +1322,9 @@ class TestbenchGUI(tk.Tk):
     def _sweep_vi_done(self) -> None:
         self._btn_svi_run.config(state=tk.NORMAL)
         self._btn_svi_stop.config(state=tk.DISABLED)
+        # Wait for sweep thread to fully exit
+        if self._svi_thread:
+            self._svi_thread.join(timeout=5)
         self._svi_thread = None
         self._svi_stop_event = None
         # Restart normal worker polling