I would unplug the A/C compressor clutch harness. It may have a loose connection. Make sure to check it for any corrosion. They are little pins and it doesn't take corrosoin to act up. Second, make sure the connection is tight. You should not be able to move the connections when they are together. If neither of these help, visually check the harness for rubbing. Start at the compressor clutch and work back into the engine harness. It may help to power up the truck and turn the AC on (engine off). Then if you are wiggling a loose or rubbing connection, you should be able to hear the clutch kicking in and out!
I guess you might be complaining of a blower motor instead??????? If so (fan kicking on and off). I would still follow the above mentioned procedures, just direct them at the blower motor instead!