I've had the same problem for some time. Recently it got so bad that it would shut off immediately after starting up. I did some research and found that replacing the heatsink usually fixes the issue. I just replaced the heatsink and it works! No more shutting off. Oh, there is one problem though. After I finished the heatsink installation, when I started up the computer, there was a puff of smoke and now the display backlight won't turn on! But at least the computer doesn't shut off constantly. Any ideas on what happened to my display?
Anyway, look into the heatsink issue.