The BIOS is very limied and does not have a temp monitoring page. The
fans are blowing mildly warm air out, and they are lowd, but sound
normal. If it ever boots into windows again I will try to install a
temp monitoring program like speed fan, and see what happends.

I do suspect it is overheating though, because like I said it hung at
POST even if it was turned off for several hours; and then later it
made it into windows. This is certainly unpredictable behavior.