Do you ever encountered a problem like this? What is your suggestion for me to try before replacing the motherboard?
Just to be absolutely sure: Did you make sure the Intel ME is in place and undamaged? It is known that modern computers shut down when the ME is touched. This should happen after a certain time, though (30 min ?), and it shouldn't occur randomely like in your case.
It's definitely not the 30-minute shutdown.
Since the first day I got the machine, the ME has been stripped down using the me_cleaner script. I also set the AltMeDisable bit later on. Although these two modes don't really have a observable difference.
Still, if you changed the IME I'd try reflashing an image with an untouched ME and test the machine.
P.
I don't think Intel ME is involved doing the black magic here, but I'll probably test to run the machine with stock ME firmware when I have time.
Tom Li