Mac Pro shuts off at start up, red light flashes once on motherboard
Hello. My Mac Pro 3.2 ghz Quad Intel Xeon has recently started shutting off during start up. It has been spinning like a top for a year and a half, and then suddenly.... boom. It happened a few weeks ago for the first time. I just turned it off, as I didn't have time to deal with it that day. The next day I turned it on and it's been running like normal, until today...which was about 2 weeks of it working after the first shut down. When it does shut down a red light flashes briefly on the board behind the hard drive (which I believe is the motherboard?).
Any suggestions before I blow a bazillion dollars and or just buy a PC would be much appreciated. Thank you!
Mac Pro, OS X Mountain Lion (10.8.5)