Your issues definitely look like heat-related... even if there's no thermal shutdown device.
Just a thought: maybe there's some metallic piece (or solder joint) which dilates
on heat and ends up shorting some power lines, forcing shutdown? The problem could reside on the power push-button
at the back of the computer...
A somewhat unrelated note: my Dad's 7500 was experiencing some random power-UPs
, from a few seconds after shutdown, to hours or even days later, even in the middle of the night -- crazy! I tried everything
described here to solve the issue, to no avail -- even the "defibrillation" procedure
...eventually, I solved the problem by replacing the ADB keyboard
-- seems like it shorted the power-on signal randomly
2xSE/30, Colour Classic, Mac II, IIx, 3xIIsi, LC, LC475, LC575, Power Quadra 700, Quadra 840AV, 2x5200, 7200/90, 7500/132, 7500/233, 7600 G3/450, PowerBook G3 Bronze, iMac DV SE, MacMini G4, MacMini Core2Duo, iMac 21.5" i5... and SGI Indy R5K