Auto Air Conditioning & Heater Repair: Silverado a/c problems, technical bulletins, long distances

Question
I have a Silverado 2002 6.6 l Duramax with some a/c issues when pulling a trailer for long distances.  On long trips the blower speed is significantly reduced and does not cool as effectively.  When you stop the vehicle and turn it off for 30-45 minutes it will resume cooling again.  We've had it checked out by the dealer and they say everything seems okay.  Got any ideas on what the problem could be?

Answer
HI Kerry

This may sound crazy, but we had one and the only thing we did to it was clean the throttle body.  This seemed to help the problem.  I still do not understand what the throttle body has to do with the AC.  All I know is we found it in one of the technical bulletins and it worked.