Head gasket hell

I'll bet that's an overdrive 4-speed, idling down the hiway at ~2100, with not nearly enough ignition timing. Maybe even with a 4-blade fan, no shroud, and the fan is like a yard from the rad right?
Is that a lean-burn truck? I bet the truck sucks gas too! If yes check your cruize-timing, my money would be on a failed/failing vacuum sensor. If not that, then check the water circulation, maybe it needs a pump.