I've recently started having the A/C compressor stop engaging after high rpm. As long as I don't run the truck hard the air will continue to work, but if I am towing something and it drops down a to a lower gear to pull a hill then the compressor won't engage any longer. After this happens I can stop at a light and turn the truck off and start it back up and it will engage immediately. Any ideas what could be causing this?

Thanks in advance.