I had a similar issue with my '94 E320T when I first got it - fine around town but would start dropping to lower gears and eventually losing drive after extended highway driving; pulling over and letting it cool down would let everything return to normal.

Not having a high level of confidence in the maintenance habits of the previous owner, I drained the transmission, replaced the filter and refilled with correct fluid to the proper level, and it's been fine ever since.

Your original post said that you had good fluid at the right level, but it can be difficult to get a proper reading on these transmissions, so starting with an empty transmission and putting in the correct amount will at least give you a known baseline. Plus, it's easy and cheap and can't hurt.

I figure mine was aerating, and stopping the car to cool let the bubbles come out of the fluid. Don't let anyone tell you these transmissions are fragile - both my wagons have/had mileage well into the 200ks and work/worked perfectly fine.

__________________
Robert Fini

'12 ML350 BlueTec, 111k
'06 E320CDI (x2)
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 72k
'97 C36AMG, 325k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)