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#11
So I actually followed your advice Compumind.
Not much dust in the case but when I tried to remove the GTX, the plastic cover over those dozens of fine pins on the MB came with the GTX.
That means that whatever system is used to fix the plastic cover over the graphic slot (gluing ?), it somehow went loose.
It was of course impossible to refix the plastic cover over the pins because there are simply too many and even if I knew how to enter every pin in its hole in the plastic cover, I wouldn't know how to fix the cover to the MB.
So I put the GTX simply in the slot beside it which was OK and the plastic cover firmly fixed to the MB.
The system loaded perfectly and since then I have no problems.
So the problem was probably due to the slot plastic cover going loose for some reason what made the contacts between the GTX and the pins random - sometimes it worked and sometimes it didn't. Just moving the computer might have been enough for some pins to sometimes break contact.