Is my graphic card unstable?

i have radeon 7870 and the core clock is overclocked to 1200 and when i play a game or turn on msi kombustor, the program turns off after several minutes and i get a "Driver has stopped responding and has recovered" message on bottom right. The temperature goes all the way up to 80 celsius under full load. So i think the overheating is not the problem...

And when i lower the core clock to 1150, it works fine...

I have seen so many people that OC the 7870 to 1200 mhz and sometimes 1250 with stock voltage.....

I have 530w powersupply. Do you think the powersupply is holding back the graphic card??