73 MGB's Overdirve

My 1973 MGB with overdirve will sometime not move when it is placed in reverse. this problem occurs when the car has been drivne and the engine is hot. The gear shift goes into reverse properly, the clutch works properly and the car goes into any of the forward gears properly. It have been suggested that the problem is a overdirve problem.

Any help you can provide would be appreciated!

Bob