Air vent blend door inoperative

there will be a air flow setting knob that controls whether air is defrost or on feet or from the console vents.

HAVC Blend Door problem under the dash

It's one of the blend door motor drive gears that has cracked. As the motors try to move the blend doors to divert the air flow, the gear attached to them slips on the drive shaft. You need to remove them, inspect for the broken or cracked drive gear and replace them. This is a very common problem with these older GM vehicles.

Possible bad blend door actuator .some gm cars can be relearned to the new climate control head

Mode Door 1.
a.This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
<!--[if !supportLists]-->4.Mode Door 2.
a.When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver's side footwells. Failure is indicated by an inability to choose between defrost and floor.
b.This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It's still better than the dealer fix because you don't have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you'd most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It's inconvenient, but you'll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.

Could be the HVAC controller, or a faulty blend door under the dash.
The controller works the blend doors to control air flow thru the heater core and the vents.

Based on your description I would say you have a blend door that is sticking or the control motor is going bad. The blend doors control air flow.

This is a common problem on this site.
In your case I would look for a vac leak or a sticky blend door.
Since the direction of air flow changes when you are accelerating, I'm going to guess the blend door or doors are controlled by engine vacuum. There may be a vac leak between the dash and the engine, or the blend door may not be able to move back and forth correctly.
Defrost is the default position when a problem occurs, to make sure the windshield is clear of fog or frost.

check for anything that may have fallen into defrost vent poss jammed air blend doors.

THe blend door seems to be stuck. When you change settings from defrost to floor air or, vent, the blend door opens and closes directing the air to the place you want it to go to. I have had some success spraying a lubricating oil such as WD40 under the dash on the blend door. You need to get under the dash with a flashlight to locate it. It looks like a flap. Spray it with a couple of shots of the WD40. Go easy on the spraying as you may be inhaling the fumes. Keep the doors and windows open to minimize the fumes. Wait about 1/2 hour and try the A/C functions. (defrost, floor, vent air)