w126 shifting problems
 

I have done several searches, and can't seem to find anyone that has had a problem similar to mine. This is my '88 560SEC. Transmission rebuild 4 years ago, shifts are firm, no issues there. The problem is erratic, but here is what is happening:
From dead stop acceleration, car runs/shifts fine. Problems occur at steady state driving. As long as I have some pressure on the gas pedal (and vacuum on the gage) car performs normally. When I let up on the gas, there is an abrupt feeling, as strong as an upshift, only that rpm drops, as does power (feels like car lost power) and I decelerate. If I push back on gas pedal and accelerate, all is normal, until I again reach steady state. Upon stopping, car idles fine.
It doesn't do this all the time, and if I pull over and shut the engine off and turn it back on, the problem goes away (makes me think a switch or sensor is stuck).
Can anyone help me, before I have to resort to taking it in for service?