RR gearbox fault on dash

Driving along sudden loss of power, gearbox fault on dash then cuts out and will not start. along come the RR autoelec does a textbook test and it showing Alarm Fault nothing to do with the gear box, spends a couple of hours and behold hes driving around the block, what was the problem I inquired, no idea was his reply in 28 yrs I have never come across this fault before, but it seems ok now. Fast forward a couple of months having been on holiday to Devon and other parts of our great land and you guessed it the same fault has reappeared. Does anyone had this fault or know whats going on???