How about grounding problems with your mobo? If you have a contact somewhere between the mobo and the case, it can cause power to ground and you can get all sorts of flaky probs.

Since this only started when you opend the case and added some RAM, I'd be checking for some sort of short (fallen screw maybe?). Remove the mobo completely, check where it touches the case (are there any "extra" metal stands touching where they shouldn't?) and make sure there are no bits of metal anywhere there shouldn't be.

Another thing that comes to mind is power supply. I know you said you changed rooms and sockets, but the power supply in the PC might have a prob, and it could be flaky. Try ALL of your original components in a completly different case with diff power supply if you can.

Hope you can get this one sorted out.

Cheers
Mick