Could it be that once the vacuum comes up that you are shutting down the engine as if turning off the key? in other words there's some sort of vacuum applied to the engine shutoff valve? I don't know if that is even possible, but you might try disconnecting the vacuum line to the shutoff if you haven't already.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)