Not sure of the problem but depends on mileage and power output etc. Im sure someone else will be able to shed some light on the problem though
But you can get gearboxes second hand ( ebay, scoobynet,sidc ) at reasonable cost and most garages can fit them, you could give sw autos a call 01580291588 they will be able to help you and i used them and there great blokes
Hope you get sorted
Tony