IT IS NOT OVERHEATING!
When I feet the heatsink it is near enough "room temp" so I took the other side of the case off and felt the back of the motherboard where the socket is, maybe a tad above "room temp" feeling, certainly not warm. Besides, if the heatsink was failing to take the heat away and so letting it get hot enough to fail during the POST, which it has done once or twice (the machine boots up to the Ubuntu desktop in about 12 seconds from mains power switch on) it would fry big time. Don't forget that it oftern runs for many hours without problem.
I am prepared to consider something in the BIOS may want tweaking, timings, volts, I don't know what in there may need setting to suit the remaining hardware........
Rans6..........