Sounds like a vacuum leak...
in one of the door actuators.
Mine were sporadic also, after replacing two definitely blown units. The third unit had a small (very small) rub spot that apparently had a pin hole leak that would sometimes seal, sometimes leak.
Other vacuum points to check, the gas door diaphragm, trunk lock diaphragm, the check valve under the hood, all the rubber connectors for the vacuum lines. Only takes a good Saturday to go through it.
If you don't have a vacuum gauge or a hand vacuum pump, get one. Approx $40.00 for a good hand pump that you can use to pull a vacuum on the system. Disconnect the vacuum reservoir from the system when doing you testing, any leaks will be magnified due to the lack of reserve.
The easiest way for me to test each individual element was by pulling the door panels rather than cutting the hard lines or fishing the Y connectors out of the pillars (or pulling the glove box to get access to the lines. If you are careful and take your time you can de-skin the doors without damage (plus you'll have to open the doors up if you replace an element).
Hope this gets you started.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.