Thanks guys. I don't have a leak that I know of. I think the problem was pilot error in reading the dipstick as I thought so long as the engine was warm the reading would be right...checking it with the engine running and after the top off yesterday and it is now consistently about 1/4 inch above the full mark. (by the way, talking directly to the MB tech back in Florida that blessed the car before I flew down and got it, reveals that despite forum comments about the tranny dipstick reading low when the car is off and cold; a very high fluid level is correct.) The car is a 79 by the way and I sure hope there's a vacuum modulator & hose as this sounds like the key. The crappy Chilton's manual gives no clue, but from reading posts, I get the sense if it exists, it is on the right side of the tranny just above and in front of the pan right? And that the hose is/was originally metal? If I don't see any broken or loose hose, is there a way to check the modulator's function? And if there IS a loose connection, is there a method to "tuning" the modulator (I get the impression it has some external adjustment knob), or is it trial and error?
While under the car tonight, I'll also double check the radiator fittings as I suppose it is possible the junk yard radiator isn't circulating the ATF properly.
It is of some comfort that since the slippage started all at once and not gradual, and that it is only during the shift, that the problem could be minor...I have my fingers crossed.