Hi pulchr, thanks for your reply.
I just installed IOQuake3 to see if I got this problem also with Quake3, and I got these same problems with IOQuake3 aswel...
So I tried normal Quake3(1.32) again to test for sure if it's only IOQuake3... and I just figured out I even got this in Quake3 now!
This is realy weird, I got this problem in Ubuntu 7.10 a few weeks ago, but even after a clean new Debian install I have these same problems...
editI think there's maybe something wrong with my hardware. I'll test Quake3/OpenArena soon in Windows also. If i got these same problems again, then I'm almost sure something's wrong with my hardware...