86 244 M46 240,000 ml aprox.

Recently when accellerating I hear this squeaking sound coming from the transmission, as soon as I let up on the gas it stops. Not sure if it occurs at high speeds but around town its happening. My third gear was slipping occasionally the past two weeks but not in the past few days. Slipping in that while cruising along at around 35-40 mph it would all of a sudden for just an instant feel like the transmission was in gear, it would do it two times or so and then be fine. I read a recent post about dirt/oil on the flywheel as a possibilty and thought that I did clean the engine with cleaner and a high power spray with alot of focus on the passanger side rear part of the engine. Any ideas? The clutch is three years old that is if the mechanic put the new kit in when I had the transmission swapped from an 83. (transmission has 280,000 miles but feels good, clutch cable is new.)