...gear when warm.? The car has just started to do this so before I have the gearbox rebuilt which was one suggestion by an transmission expert, has anyone else had this problem or could offer any advise? The car is 1997 e39 with a tiptronic sport mode.

Did the transmission expert look at the transmission fluid level and condition? If the level is ok but looks black and smells burnt then it is time to get a new or rebuilt transmission. But if it lacks fluid then it is worth the try to top it off first and roadtest before condemning the tranny.