Disconnect all power to the mobo, so 20/24 pin, 4/8 pin and 4 pin molex if that's plugged in. Then use a jumper to clear the CMOS, make sure the battery is in at this stage, wait 5-10 seconds and then remove the jumper. Then for good measure remove the battery for a few minutes. Then put it all back together but this time only plug in the 20/24 pin and see if it'll boot, then try with the 4/8 pin connected and lastly the 4 pin molex.
The above sounds a bit anal but it's roughly what I had to do to get around a continual restart/shutdown problem. Also make sure you've only got 1 ram stick installed.