First, if you shift into low or reverse, and the car is not moving, does the problem occur then? If so, it is a torque converter issue, probably in the lockup area. At idle, in any gear, and the car not moving, the input shaft should not be turning, and torque converter slippage should let the engine idle with minimal drag. Having the trans scanned may show up that the computer is trying to lock the converter up for some reason. Second, if the problem occurs as you are moving in manual low or reverse, then it probably is either the low/reverse BAND (there is no low/reverse CLUTCH) and/or the over running clutch (rear sprag) is failing. Both would have to occur for the problem to happen in both rev and man low. Even if the low/rev band was slipping the rear sprag should hold for a clean first gear. A better description of what is happening is needed.