98 1500, under heavy acceleration the AC discharge goes from the dash vents to the defroster vents, to the floor, and then back to dash vents. What would be causing this? Thank you.

 

Either a vacuum leak or a bad check valve.

 

Check the vacuum line that comes from the passenger side of the engine intake manifold. Follow it where it tees off and goes up over the PCM towards the drivers side, I have found several bad deteriorating lines. Personally I just replace them now instead of just looking for cracks/splits. Leave a short length at the tee port and near the fire wall. Have an auto parts store clerk match up the size and route the new one and connect up to your short pieces. That should restore the proper AC operation of the vents.