In my case, it was VGA first, and then PSU. I think it's tricky to find which one is a real culprit. Even if your VGA is fine, when PSU cannot provide enough power, it can be shut down. In this case, using onboard graphics may alleviate the problem temporarily. I would try a new PSU. If this solves the problem, stick with it. If the issue persists, then get a new graphics card.