Im not an expert with benz's, but i had the same problem with my 500 sec, also a slightly rough idle. Everything looked fine, tryed adjusting the modulator, but did not help very much, after a few hours of fiddling with this thing (and a few choice words), it turned out to be a small, tiny slit in the vacuum tube, i only noticed the slit after taking the hose off and throughly checking it for about 30 minutes! It was so small, i could hardly see it, but it was enough to lose vacuum. I replaced the tube and the trans shifted perfectly, nice and smooth from park or neutral into drive or reverse. Like I said, im not an expert, but that was my experience with the trans modulator!!

Good Luck!