Other than the usual stuff, try to run the tractor in all it's gears. Watch for it to jump out of gear, usually third or second, especially while going downhill or slowing down. This is the biggest weakness of the D-series tractors. If they are worked hard in second or third gear it tends to wear those gears and their associated parts to the point that this occurs. The fix isn't that hard, but it isn't cheap either, moreso if you have to hire it done.