I can reproduce the issue each time.
They appear even the Amiga cold at first start.
Now when I launch Fblit, they don't comeback anymore.
But is the real solution ?
As I said I use the HighGFX driver @ 1024x768.
Maybe you guys are using ClassicWB with fblit ON and so the glitches don't appear ?
Can you test without fblit and my screen resolution ?