There are several sensors located on and around the radiator stack. I would start there and work towards the firewall along the drivers side frame rail for the AC pressure switches. Check the connections, terminals, etc. Check the harness for the coolant tank sensor on the passenger side, I've had an intermittent issue with that sensor and harness.
Check your grounds... there are several behind the kick panels under the dash, I would recommend actually taking the grounds off, cleaning the terminal and reconnecting. Check all the grounds on the truck. I solved a bunch of little issues this way. 1998 Volvo 610 Cummins engine 10sp.