I have a '97 F-150 X-Cab Lariat 4x4 with a 4.6L Triton, and I am having the same problem as powerlifter-- A/C (haven't used the heater so I don't know if this occurs using the A/C only) is on and all of a sudden all air comes out the defrost vents. However, my problem occurs when I am accelerating. While accelerating, I can ease off the throttle and air returns to "main" vents. Problem does not occur during "cruise mode"(a constant speed I mean) either. Does anyone have a suggestion why this is occuring and how to fix?? THANKS

 

Quite possibly the inline vacuum check valve into the cab is bad. On hard accel your engine vacuum drops. The heating system is designed to default to defrost if vacuum to the system is lost. So for some reason you are losing vacuum to the mode door actuator. Also check your vacuum canister.