I have a 1995 F-150 4x4 ext cab and a 1999 F-350 4x4 super duty with both the same problem....here it is....when the AC is on it works fine until I hit 2000 RPMs then the AC blows out the defrost until I hit about 1700 RPMs then it blows out the panel like normal. Need some help to track this down, Thanks
Troy

 

Look for a vacuum leak in the system. Start at the vacuum cannister under the hood and go to the control head. Also look at the little actuators under the dash on the heater/A/C plenum.