It could be an overheating problem. Can you monitor your board and CPUs temperatures? It could also be that the plugin your using w/your card isn't exactally compabtable either. Or maybe you need new drivers for your card. Why not post some system specs and we'll see if we can help you out better.
I had a glitch like this up until recently. I eventually tracked it down to a glitch in my with my mobos agp drivers. If you have a via based mobo try installing the latest 4 in 1 drivers. If that doesn't work then install the latest 14.x detonator drivers and from the hardware tab in the advanced settings dialog disable agp memory access (by moving the slider to the left). Although this will slow down the card its better than having it crash right.