PC Transplant Pain

Oh dear. I ripped out the guts of my PC home Ubuntu server and replaced them with the old guts of my main desktop PC which I upgraded recently.
This all seemed to go quite well, the guts-ripping was completed in style, and the dust-cleaning of the existing case completed to satisfaction. Then I put in the power supply and motherboard from my old desktop. The mobo still had processor, heatsink, and RAM attached, no big deal. I used the existing hard drive to keep the Ubuntu install as my network server, and added some extra SATA drives from my old PC, made redundant by the awesome new RE3 RAID 0 twins.
Started it up, and after some initial BIOS shenanigans, got Ubuntu up and running nicely. Then. After 10 minutes. CRASH. Black screen, PC speaker waiting WEEE-WAH-WEEE-WAH like a desperate siren looking for a wreck. I don't think it's heat as I've checked the BIOS heat levels for CPU and mobo and added a case fan. Its not the video card, because I swapped it with another for the same result after 10 minutes. How can it be the mobo/RAM/CPU after I've been using it for five good years??
I don't know. I have sent mail to ABit tech support hoping for joy.

Bugger.