Just had a very similar problem with my 87 300SDL (same body as yours). Problem occured after an A/C repair, and the problem was the rubber vacuum connection between the hard plastic line, and the transmission modulator had disintegrated (guess they "bumped" the hard plastic line while replacing the A/C hoses). Discovered several connectors between hard plastic lines and vacuum devises were soft or cracked, so I replaced all of them. Everything works great now, with a cheap and easy fix (transmission also shifts better than it has in years, so some of these may have been leaking for a while).

Fix the vacuum source first, even if it is an actuator, you still have to have a strong source to drive all the actuators.

Good luck