Usually, when you have a [problem like this, you have either a bad vacuum motor or hose with a leak.
What is extremely helpful is a hand held pump up vacuum gauge. Auto parts stores or harbor freight has
What i would do is disconnect the vacuum line feeding the HVAC
under the hood and suck it down under the hood.
If the level does not not hold vacuum on the gauge, you know you have a leak under the dash.
I had one bull i worked on that the hose under the hood feeding the HVAC
was bad and leaking enough to
cause function problems.
The method of test is once pumped down and stable, it should not leak down.
You can do this test on the whole system or individual vacuum motors.
On my 90, i had a leaking vacuum motor that caused the air flow to shift from
defrost to other vents as i reved up the motor.