S80 D5 Autogearbox loses drive when it warms up
 

I have tried to look through the forums posted messages and resolutions and could not find my problem.
I have a S80 D5 automatic on a 55 plate registration.
I have had a second hand gearbox fitted.
From a cold start the vehicle drives beautifully, and as soon as the transmission warms up, it loses drive! Let it cool down, and it drives off again etc.
The garage has run the diagnostics and no fault codes appear!!!!
Can some clever person out there help me out with advice or better still a solution. Thank you Regards Ted (user name Rocks)