thanks for the advice, was on the xbox support page and to fix an out of waranty xbox360 is £68 but as i said not to sure if they would fix it as i have had a go my self.
I do have a new slim version now, just trying to get this one fixed for my son.
well after doing it a third time with artic silver 5, it has lasted all weekend and still working fine.
think its a lot to do with Mother/board flex and how much or little you tighten the fixings on the cpu and gpu and also the outer case fixings through the board.
fingers crossed.
cheers guys.
UPDATE.... it has gone againtime to send to microsoft