Did it pass memtest? If it passed with no errors try running prime95 to see if it fails that. If it fails prime95 with multiple threads within three hours its probably the northbridge(motherboard problem). Make sure to run "hardware monitor" found at
www.cpuid.com to see the temps of the components while you run prime95. If anything passes 80 degrees Celsius you have a cooling problem.
BTW are you still using the original 9600gt Sonic it shipped with, or are you using something else?