Okay. My first suspect is overheating.
Download HWMonitor, Prime95 and Furmark. http://www.cpuid.com/softwares/hwmonitor.htmlhttp://files.extremeoverclocking.com/file.php?f=205http://www.ozone3d.net/benchmarks/fur/
Launch HWMonitor and Prime95. Go to task manager and set prime95 priority to low (processes tab). Then, in Prime95, start In-place Large FFTs test.
Next, launch furmark and start burn-in test. Switch to HWMonitor tab and watch the temperatures climb. If they reach 80°C for the CPU or 90°C for the graphics card, shut down both prime95 and Furmark. If they don't do that after 10 minutes, take a screenshot of HWMonitor window and post it here.