It has the 722.9 transmission. It also has an ISM, which actually shifts the transmission as there is no traditional shifter in the cabin. It sounds like an ISM problem, but there is a wide range of failures that can cause similar symptoms. This isn't something you're going to diagnose and fix over the internet. Even if you can determine that the ISM is the problem, it is a theft relevant part. That means you'll have to jump through hoops to even order it and it will have to be programmed over the MB servers and married to the car before it will even function. Your best bet is to take it to a highly qualified Mercedes shop, or barring no other option, the dealer for a competent diagnosis. It is quite likely going to be an expensive repair and guessing will only add to that total.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff