Thanks for the answer, but I dislike throwing whenever possible, and decided to go as far as I can to find from where does the error come from.
Maybe is the GPU not correctly soldered or something similar.
I unmounted the motherboard.
Here are commented photographies the recto and the verso of the motherboard:http://imgur.com/93XP2http://imgur.com/nVdEe
I would be pleased to hear from other people who dissasembled their T42.
If they can remember visible differences.
Also, are there opinions about POST diagnostic cards. Are there some efficient ones for the T4x?
Lastly, does someone have an idea about the price of a correct reflow equipement and supplies? I can imagine some equipments being very expensive, but what about one to unsolder/resolder main chips like the GPU?
I see this as a long-term investment.