Originally Posted by Genbiltstein

Throwing a wild dart at you. Is the transmission downshifting correctly. Downshift manually to first gear. If you are not stalling then we need to look further. Its a 1988 mercedes. Count the gears as you shift all the way to cruising speed. If you have four gears then I am going to wonder if your torque converter is coupled correctly when you are in the lower gears. Hydraulic coupling is necessary. Mechanical coupling is used to cool the tranny in higher gear. If the mechanical coupling is not released into hydraulic coupling then therin lies your problem.
Coupling from the engine to the transmission at idle is only possible in hydraulic coupling. (torque converter)