Again. Thanks a lot. Good experience on the forum with this funny weird problem.
I was able to transfer data from encrypted drive running old software on computer. Took ages as computer took forever to decrypt.
The construction really flimsy and I am scared to ruin it by taking it completely apart.
Therefore I have not managed to to go on with the the proposed solution regarding the discharge MOSFETs, as this problem suddenly did not persist as I changed something else.
KICKING stupid fan control!
Fan control weird!
If CPU is cold and heats up even to excessive temp, then fan control does not bother at all to engage.
But if CPU was totally overheated when turning on, then suddenly I get fan spin for the whole session, even temp is below threshold after some time.
WTF.
So overheat CPU first, then reboot, to brutally cool it down agin? Not a good idea for long CPU lifetime I guess.
Fan controller must have issue, so I went to turn on fan continuously right from start. Just by shorting reverse voltage discharge diode.
Works just fine. Computer louder, but works for the purpose much safer and more reliable. Happy. Hack, I know.