First off, your PC temp is NOT your GPU temp. You should have a CPU temp, motherobard temp, and then usally a chipset or two and your harddrive.
As for a heat issue, I saw my old 6200TC reach around 70*C before and it never messed up, so I seriously doubt heat's an issue, at least with your GPU. Their max temp, if I remember correctly, is something around 115*C before they begin clocking down. Check your other temps...