Any Ideas C5 Auto Box

[coastline taxis]

c5 2001 2.0l hdi 185k automatic

car developed a gear fault last night on the transmission it wont change gear and crawls along at 2-3 mph this was after 2 hours of running driver said the the display read automatic gear fault. Plugged car into ethos scanner went to gearbox and no fault codes found. checked the oil level in gearbox and it seemed fine this took about a hour in total. went to take car out of garage and hey presto everything was fine car drove as normal so put car and driver out to work. 20 mins later came back with same fault only this time drivers say there was a horible burning smell so car parked up for the night. Went to car this morning and drove perfect then when warmed up back came the faults. so drained and refilled the gearbox car drove fine then when warmed up fault back. So we then disconected the battery left it for a hour and reconected it car drove perfect for a hour this time then wouldnt drive done battery again and car drove perfect also works in sports mode but wont go up hill in sports mode ( its feels the same as when the clutch has gone on a manual box loads of revs with no movement) so in a nutshell reset the battery and car runs fo a hour then has to be reset or you loose transmission. cant complain as the box has 185k on the clock so had me money out of it was wondering if anyone has any ideas

Was thinking of putting a manual box in as i have a car for spares so all parts are there if so would the ecu need to be altered would it show faultcodes up if it didnt get the autobox sensor inputs or would it just run as normal or does it have to have another auto g/box put in

Any comments or thoughts greatly accepted