I can almost guarantee that the changeover valve has bad solder connections. I've picked up a couple at the wrecker and they as well as the 2 in my W124's had cracks where the solenoids solder to the circuit board.
Later valves can easily be disassembled by just undoing screws but the earlier ones can be a challenge to get apart and back together. Not sure what year that changed but 93, 94 and 95 definitely have the screw together version. If you have a wrecker near you it would be worthwhile picking up the newer one, re-soldering it (add fresh solder) and installing it.

As Diseasel300 suggested, check you vacuum source.