I can't remember for sure, but it has something to do with the DSP or some bus in the area.
When I connected my SCSI ZIP drive to my falcon, it worked great. Later I installed a CT60 and I had all kind of scsi problems. things went worse when using higher resolutions (VI, Centscreen, you name it). Then mr. Czuba asked me to run some DMA or DSP tests, and they showed some malfunction in those areas. Stuff was fixed at hardware level.
However, still now, if I run those tests in very high resolutions, I have some glitches.
Lesson learnt: too much stuff into screen=bus clogging somewhere (and DSP getting hot). I believe everything will be fixed with SuperVidel or CTPCI
So, to resume things, I think you should check quality of ram, DSP and DMA 'wiring' and set a suitable screen resolution