This problem seems quite reproducible.
1. Set file caching parameter very high -- e.g., 30000msec = 30 seconds.
2. Start video.
3. Open Message window & set to debug.
4. Increase speed to perhaps 1.8x (hit "]" a number of times).
5. Wait until video & audio are playing smoothly (more or less).
6. Stop video with space bar.
7. Start video with space bar.
The VLC control loop goes crazy trying to restabilize; it continues to flap.
Here's how to get out of it.
a. Stop the video again.
b. reposition video to a different part of the video -- even a few seconds change will work.
c. restart the video & it plays ok.
Apparently, by repositioning the video, it clears the buffers, so the control loop restarts from scratch.
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Suggestion for fix: when a video is stopped & restarted, treat this situation as if the video had been repositioned.