I have the same mobo and cpu but a geforce vid card and pc2700 memory. I get the same problem if I unplug mains and take the side off for whatever reason but it only happens once after which all is well. When the problem arises I get 2-1 beeps which suggest graphic card problem then a series of short beeps (about6) which is a puzzle. Like you I have to switch off by holding start button after which I have no further problems. I must be honest and say I don't know what causes it I have one or two theories ranging from a late charge up of a capacitor or HDD not getting up to speed quick enough. I fitted a 400w psu but it made no difference. I still suspect that unplugging from the mains is a pointer but can't get my head round the reason. My cmos battery is ok as far as I can tell and get none of the indicators that would suggest a problem such as checksum errors. I know this doesn't help you but some other wizzkid may put his/her two pen'orth in.