Volvo Repair: 1988 Volvo overdrive(transmission), overdrive transmission, reply thanks

Question
 My overdrive (740 Turbo740 wagon) slips in and out at speeds usually lower than 55. The overdrive does even engage at any speed many times, especially when the car has been running. Usually, when it does engage, it 'slides' in.Many times, it will seem to get caught between 4th and overdrive not wanting to make up its mind what to do.The overdrive light always works when I push the button and I always use the clutch properly.The clutch, by the way, is new .This problem began occurring a couple of years ago, however. The mechanic told me I need a whole new overdrive unit.I know there is a relay and a solenoid which sound like they might be worn or faulty.(They are the original parts.) I look forward to your reply. Thanks for your help.

Answer
usually when this type of overdrive has a problem the best result is a new unit or rebuild the one in the car.they can be rebuilt but I am not sure the parts are still available since I have not done one in a long time.it could be the solenoid but sounds more like a pressure problem due to bad seals in the overdrive unit.