It's not thermal runaway from what I can tell.
I used an infrared measuring device on the heatsink and it showed roughly 30-35°C. When I touch it during a benchmark it feels around 35°C. This is also what the BIOS says.
Here's the results of my tests with 3DMark 2001 SE (default benchmark):
1Ghz: it does all the tests and then crashes right after
750Ghz (FSB 100): it does all the tests and then crashes right after
500Ghz (FSB 66): it does all the tests and then crashes right after
--> then I swapped to my 800Mhz CPU:
it does all the tests and NO crashing.
Sure the voltage is higher but it's what, 0.05V higher? It says 1.75 for the 1Ghz and 1.7 for my 800Mhz. I could even raise it to 1.75 on my 800Mhz and see what happens but I bet it will run just fine.
Also: the heatsink is on correctly - originally the metal lever on top was oriented the wrong way (the lowest point is supposed to be directly above the CPU die and it was the other way round) so I corrected this.
With the current high speed fan, the CPU really doesn't get warmer than say 35-40°C tops.
There's 21 caps on this motherboard (I counted them) so I'm not very anxious to replace them when the 800Mhz CPU works fine. I still feel it's unlikely to be either the mobo or PSU since the PC is rock solid on the 800Mhz. I mean, I used the 800Mhz to 7ZIP multiple Gbs of CD images at ones (I'm making 1:1 copies using Alcohol 120% to run off 128GB USB sticks - way faster load times and no CD drive noise!) and it worked fine even though this would mean the CPU is maxed out for ages at a time and it did it just fine.
Retro game fanatic.
IBM PS1 386SX25 - 4MB
IBM Aptiva 486SX33 - 8MB - 2GB CF - SB16
IBM PC350 P233MMX - 64MB - 32GB SSD - AWE64 - Voodoo2
PIII600 - 320MB - 480GB SSD - SB Live! - GF4 Ti 4200
i5-2500k - 3GB - SB Audigy 2 - HD 4870