Thank you again for your quick answer!
I guessed it has to be a physical HW issue, as it happens regularly when the PC is moved or slightly twisted (like pulling one corner up).
I was mentally ready
) to re-ball the GPU. However, I am not sure about the SB, can it be done in a home environment?
In any case, light pressure on the GPU did nothing. Nor on the SB, or elsewhere.
I dismantled the whole thing, looking for some loose part, but I could find nothing. I assembled everything, restart, and it worked for 24h no problem. I could even do the upgrade to ubu12.04.
Moving the screen did nothing any more, even lifting and putting down was ok. However, when I applied pressure on the bezel, on the palm rest, everything was back again. I mean the back-lighted black-grey screen. It switches from black to grey, CD checks for disc, couldn't stop it by applying pressure. Just restart...