possibly the teeth on the shift clutch are worn, it will slip out of gear if so. The actual reverse gear does not usually wear out, its hardly used. The clutch is located in the lower unit. Dissassembly is the only sure way to tell.
If the lower unit has been swapped, the earlier models had a 3 jaw clutch and later a 6 jaw. If a 3 jaw was installed the shift throw is different and shift rod need to be changed....