2006 GMC C2500 Drivers side blows hot

The air conditioning in my truck on the drivers side switches to full blown heat while driving down the road. The only thing I can do to make it reset is to stop and turn the truck off. It might do this 4 or 5 times in a 30 mile trip or it might not do it at all. It cools fine when its working so I dont think its low on freon. What could be the problem?

Thanks
Cade