Agreed, a heat problem. You could try shoving in some extra case fans (if you don't have enough - the 12mm ones are good) or maybe look at an aftermarket cooler for your 9800gx2 (not sure if there are any for that though).
A nice big case will also help with cooling. Oh and make sure you have a decent psu too as this will help with the stability of your system
Here is a very good app for monitoring your temps (not just gfx):http://www.cpuid.com/hwmonitor.php