2005 Chevy Silverado HOT air...sometimes

Hi,
This truck has the 6.6 diesel engine.
Sometimes...about once every two days...the driver's side air (just that side) will start to blow hot....really hot. Hitting the a/c button does nothing. Moving the slider does nothing. Usually turning the truck off and starting again gets the air cold as usual. Sometimes it will start hot, but usually it will just change after the cold has been blowing for a while.
Ever seen this before?

Thanks in advance!