My voyager is making a whining sound and not changing into 3rd gear. Then it will jump out of gear when standing still and jump into gear when reved. Only when this noise appears does it have this problem.

Fluid has been checked whilst running and is topped up. I have been told by an auto gearbox garage that it could cost up to £1400. This noise and problem has only started since a garage put a new belt on. Could the tension be causing the problem. Im taking it back to the garage today but dont hold out much hope with him as he's not chrysler mechanic. Any advice appreciated