I had the same problem..."92 f150 300/6 4wd. There is a vacuum line that controls the air diverter. On acceleration, the diverter would loose vacuum and the spring loaded door would change the air flow and it would come out the defroster. I discovered a small pin hole in the line and that was all it took to cause this problem. It was near the heater plenum under the hood...very easy to get to and fix. THOSE CONFOUNDIT HARD PLASTIC LINES ARE THE WORLDS WORST WHEN THEY AGE A BIT!