Motherbaord problem. computer Shuts down..please help!!

Well I had this problem a few times. I hit the power button and everything seems to start up normally. Then it gives me 1 long beep followed by 4 short beeps. The codes on the LED when I hit the button say .3 then 8.b 9.c then I think it says f.f and shuts down. It starts beeping when it gets to the 9.c. I messed around with the switch on the back of Antec power supply 550w, turning it off and on and holding down the power button (did this about 50 times) and got it to start up again last time it happened and then everything was fine. However for the first time in a mother or so, I had to unplug the computer for about an hour and when I plugged it back in this time it started with the problem again. I believe this is the reason for it happening the 1st time about a year ago. I know its an Abit mother board with 2 gig ram and GeForce 6800 ultra.

Thanks for any help!