Ford Transit

I have a Ford Transit 125 280 mwb on an 03 plate. It seemed to have some gear change problems, so the clutch was changed, then it broke down coming out of the garage. Every time it reached a certain temperature it cut out. Took it for diagnostics but they said they could not access the computer to diagnose the problems. Suggested replacing the computer at a cost of £1800 although this may not be problem and whatever problem is may blow the new computer. Any suggestions?