I have a 2009 Mac Pro, 16 gigs of OWC ram, SSD hard drive for my boot drive and latest version of Lion. I also have an upgraded nVidia graphics card. A few months ago, it did the same thing, here are the symptoms: After a few minutes of use, system just shuts down. Power flickers on and off for a while, and it turns on and off. After unplugging, it will work again for a few minutes and then shutdown. I changed the outlet to where it was plugged to, and that seemed to fix the problem for a couple of months with no issues. Then after a really hot week, the problem came back again and it keeps shutting down. I already tried running it without the video card, no use. Is it the PSU? Why did it work after I switched outlets the last time?