Okay, I give up. Someone help.

My iMac (Snow, 600MHz, 40Gb HDD, 256Mb RAM, ATI Rage 128 Ultra), crashes whenever anything tries to use OpenGL for more than 5 to 10 minutes. It makes no difference whether it’s OS9 or OSX. Tried 10.1.3, 10.1.5 and 10.2. Crashes under all 4 OSs. Even the OpenGL screensavers crash the damned thing. What in the hell is wrong with my iMac?