When I run DF in fortress mode, it loads and runs just fine for anywhere from twenty seconds to two minutes. Then it crashes in a way that corrupts the GUI. The screen looks something like a checkerboard, cut into squares and with a colored bar at the bottom of each square. It's as if the pixel map for each square has some random noise added at the bottom. A similar set of squares forms around the cursor. For about thirty seconds after the lockup the cursor continues to move and the sound continues to play. Then the whole system locks and the screen goes black. A few times the system has rebooted, but usually it just freezes on black. The only way out is to push the reboot button.
And here's a weird part: The first reboot doesn't fix it. After the first reboot the screen stays black, though the computer mostly sounds like it's booting up normally. I have to push the reboot button a second time before the screen will show something.
My graphics card is an nVidia 6800. The problem appeared under an older set of drivers, so I installed to the latest set, released just a few days ago. The problem is still there. I completely uninstalled all grahpics drivers and reinstalled the latest nVidia drivers, and it didn't help.
I downloaded Kobold Quest and got the same problem after maybe three minutes. The problem appears whether the game is running or paused, and whether it is fullscreen or windowed. It also happens with the sound on or off, and the sound card drivers are up to date.
Another weird twist: At one point I couldn't get any signal to the monitor at all, no matter how many times I booted. So I turned it off from the button on the case and switched the monitor cable to the other output port on my graphics card. It seems to have helped, in that I can now clear up the problem with only two reboots.
Before this, I had been having a different but similar problem with Laser Squad Nemesis, where the whole screen would just go black off and on every few seconds at random. That's the only other game where I've had this problem. The only thing I can think of that changed before this started happening was that I switched from a CRT monitor to a flatscreen, and consequently my desktop is at a much higher resolution than before.
Any ideas?