loss of airflow

I have a 98 Monte Carlo with a 3100 engine. When I have the heater or air conditioner and fan running,the airflow shuts off when I hit a grade or speed up. I think the fan keeps running but the door closes shutting down the flow.
When the engine is no longer under load , the airflow resumes. Car still heats and cools but it is annoying. Is that air door controlled by vacuum? If it is that would explain why it closes under engine load. Anybody ever have this problem and know how to fix it?

HVAC dampers are controlled by engine vacuum. There is a vacuum reservoir in the LF fender of most year Montes. There is also a check valve in the vacuum lines to prevent vacuum loss when the engine is under load. Inspect the vacuum lines from the engine to the reservoir, and from the reservoir to the interior.