1990 Mercedes Benz 300te Check engine light always on

Despite having had a major service, a new transmission, a replaced BDU, O2 sensor, Cam sensor, I still have this problem.

Normal use around town at town speeds fine. However, when I go on the freeway and stay at higher speeds for a longer period the car rev counter plummets as I come off the exit and stalls. I can immediately re-start it but the tick-over goes lower and always feels like it wants to stall. Have tried endless visits to the shop with no cure as yet. The car runs fine in all other respects and is in great condition. Just need to cure this problem. The check engine light is constantly on and even after it is reset it usually comes back on within a day, followed by more stalling.

Any Ideas? Help!