Here's the run down. My truck ran into a snow back about 2 weeks ago and now the heater doesn't heat until I rev the engine above approx. 2000rpm. If the truck is idle the heater will blow cool air even when the engine is completely warmed up (half way up temp guage). Once I push the accelerator however the heater works just fine. Also once I have driven the truck and then let it sit again, the heater will continue to blow hot air for a minute or two before going back to blowing cool air. I can't for the life of me, figure out what is causing this. Any help will be much appreciated.

-Greg