98 Ford 8.3L Cummins Diesel Boom Truck Overheating help
 

We have a 1998 Ford 8.3L Cummins Diesel Boom Truck we use for building. We just bought it used and it has been overheating since we bought it. We have changed both thermostats, rad seems to be circulating. No water in oil.
We have noticed that after idling for a few hours the heat gauge spikes to just below the hot. Stays there for 15 minutes and then drops back down to the middle of the gauge.

Anything come of this heat trouble? My hunch would be the fan clutch.