I have experienced a problem for quite a while now. When I switch from X to a consol and then try to switch back my screen goes black. The only way i can get back into X is to reboot my computer. I have the same problem when I exit from my window-manager to xdm, when xdm restarts it goes black.
When I reboot my computer and x starts I my last x-session, like a screendump before xdm starts.

Also all games with the same graphics or above, SuperTux are almost imposible to play now, it worked fine before, but now my fps is under 20 sometimes under 10.

I have tried to reinstall my dist. from start and that solves the problem, but after a few weeks the same problem is back. And I have no clue what the hell is wrong.

I run debian, kernel-image-2.6.11-1-386, and my window-manager is fluxbox.

I've experienced something like this before. In my case it turned out to be lack of free space on the root partition, or /tmp if it's separate. Check into that by running df -h / or df -h /tmp

Window managers and graphic application, including games, write a lot of stuff to /tmp and . directories in the user's /home. Try to keep /tmp clean, especially if you burn or copy discs and especially if it's not it's own partition. Same for the user's /home