If adjusting the modulator made no difference it could be leaky. I thought a simple vacuum check could suffice on them for this but am not sure. I suspect it has a diaphragm but if ruptured transmission oil should appear in the line perhaps.

Something is probably going on if adjusting it made no difference. I guess a test might be to tee in a vacuum gauge between the spaceship and the modulator as already suggested.

If vacuum does not vary it is probably the spaceship. If vacuum remains flat and low perhaps the modulator is leaking. Personally I probably would grab both parts off of a junker and substitute them one at a time.

Since some suspect it is an intermittent vacuum pump the transmission should change shifting sensations. My suspicion is if the transmission issue is constant it may not be the vacuum pump itself.

On the other hand loss of the power brakes periodically and random shut off issues does sound like general vacuum issues as well. Makes me suspect you may possibly have more than one issue in the vacuum system.