i'd say either the check valve is bad or the vac supply hose is sucking shut.
pull the hose and plug it and see if the engine runs differently to check for a bad booster but normally this doesn't come and go like you describe.
i'd look at adding an aux vac pump and a vac reserve tank to that combo. the majority of 400's with the ra4 cam i've been around don't carry alot of low end vacuum. this can also cause the problem you describe.
mike
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so many pontiacs, so little time..................
moderator is a glorified word for an unappreciated prick..................
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