1997 Air conditioning and vent stop or cycle while driving

Hello
I have a 1997 Pontiac and it seems the air or vent that come through the vents in dash
seem to blow for a while then stop, but I still feel air come throught the defrost vent and heater vent? puzzles me why ? anh help would be appreciated.

The air blows from defroster openings when the engine is under load right?. This is because of a vaccuum leak, those vents are vaccuum controlled.

Inspect the accesory vaccuum line coming from the back of the intake manifold, it is a small nylon hose which runs along the firewall to the passenger side and towards the battery. Make sure it's not disconnected or broken.

Oscar.

Oscar is correct. If the vacuum lines are ok, you need to inspect the vacuum storage canister under the dash (it stores vacuum for the HVAC system for hard throttle/low vacuum situations) to see if it's cracked or malfunctioning.