Grab prime95 and do a torture test. This should rule out CPU/RAM as the cause. I've had similar problems on other machines, and it ended up being NVIDIA drivers. Unfortunately there is no fix for this most of the time, because they do not test their drivers extensively on older hardware, and rarely listen to any reports about issues... that's how it goes. You could also have some settings improper in your BIOS, make sure everything is set to the Specifications of your particular HW (CPU, RAM) and don't rely on "Auto" settings because these are often incorrect.