2001 Dodge Truck A/C blow cold and hot

I just bought a 2001 Dodge Truck for my 16 year old son and we noticed that the a/c will be working fine until we hit a bump in the road and it starts blowing hot. It sounds as if a vent door closes (or opens) when this happens. The hot air seems hotter than the outside air tempurature. We turn it off and it works okay after a minute. We have also tried turning the selecter to heat, then to defrost then back to a/c with occasional success. Can you advise me on what this is or how I can further troubleshoot the problem?