Since this is occurring while you are driving it would eliminate the fan clutch. So it is related to circulation and as Bill suggests the thermostat is most likely the culprit. Stock thermostat is 203F which is the middle of the guage.
2003 RCSB, 2wd, Street Truck
email@example.com firstname.lastname@example.org Support vehicle- 2007 QC G56 4x4 diesel