Thank you for the suggestions.

I basically got over (got used to) hidden files in MorphOS, it's ok.

I've set allow framedrop to yes and set synchronisation factor to 30 but sync problem still occursafter one or two minutes.

I guess, a Mac mini G4 (with 32MB VRAM models) will never be able to play 720p's. Even a fast OS like MorphOS doesn't seem to help it.