Overdrive disengage

I have a 1983 Supra. The manual transmission works fine except for overdrive. If overdrive is engaged it slips out of gear sometimes. I notice if cruising (no change is gas pedal) is works most of the time. If I back off the gas pedal the shifter will move slightly. If I hold the shifter in position it will not disengage. The transmission and rear end were worked on 3 years ago. Full blown rebuild on the tranny, new seals on diff. Any info/thoughts would be appreciated!!