You have a heavy vehicle with an anemic motor. Anything operated by vacuum
will eventually fail once manifold vacuum goes to zero (wide open throttle).
This can be delayed by using a vacuum tank fed by a check valve. This was
common practice in the fifties with the vacuum operated windshield wipers in
use then. Of course any vacuum leak exasperates the problem.
If you can hear a flap or something shutting in the vent system then it is
probably a check-valve of some sort or a loose vacum line. I had a similar
issue on my 99' ram 1500 4x4. Every time I acclerated hard the dash vent
would shut down until I eased up on the throttle and the front axles for
the 4-wheel drive wouldn't engage. My problem was a vaccum line that
connected the auto locking 4-wheel drive and the a/c vents was
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