I have an imac 9.1 that keeps shutting off.

Ok, I have a used iMac I picked up and for what ever reason it would shut off quickly. It came with snow Leopard, I reinstalled the OS and still the same results...I then upgraded to Lion same results. I have had computers all my life and I can't say for sure but it seems to be a video driver problem that caused the system to shut down so that it wont cause damage like fault protection. I then installed Linux on the iMac and it works great again, that is, as long as I don't enable the NVidia driver...so I am just wondering if Apple has done something different with there video drivers because I get the same result if I boot the iMac up in safe mode using the Apple OS. Any suggestions. I don't think the computer is faulty it most defiantly is a driver problem and it seems the drivers are built into the Apple OS.