I've seen a few threads in here about the vents that switch back and forth from the setting its on to the defrost mode upon acceleration. I am having that problem too, but havent found the answer to it. I too think its a vacuum leak however im not sure if i have a bad heater core as well. My heater blows very mildly warm air for about 10 sec at a time and then go cold. I'm assuming there is a vacuum that opens and closes the doors to the heater core and all the vents. I was also told to install a check valve in the line that runs into the fire wall to the HVAC unit.. however previous threads stated that solution only worked for a day or 2 and then the problem persisted. HELP or any tips will be appreciated .. THANKS A BUNCH 1996 Dodge Ram 1500 reg cab 4in clayton lift 3/4 ton durability and 33 Grabber AT2s.