Hi, first of all, thanks for the answers ! @ Joe Ellis You were right, I've forgotten some details: First I'am using a 'classical' ;-) CRT monitor (21") from Sun, works fine with brilliant colors and a very clear screen. It doesn't matter if play Quake or doing something else. But the original Apple ATI Rage Pro Card shows the same green interferences, although not so much, I think. Therefore I thought it could be a softwareproblem. I've heard of, that this is a common Apple problem ?!? @Lavode: > Sounds like it's time to troubleshoot. Or like installing Linux, just for watching DVD video :-) Thanks, Udo