The ACC system won't hold a vacuum for very long when the engine's off. There's a reservoir, but it's more to tide the sytem over during driving transitions, I think.

There's a little filter about 3-4 inches up along the small line at the vacuum pump. The ACC system will act strangely if the filter is clogged. I don't know if it can be cleaned but it's easy to replace or remove for testing. If that doesn't do it, then I'm pretty sure it's bad actuators in the ACC system.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD