I had a condition like this in my '89 3.0L. The cause was too much Bars Leak Head Gasket Fix - which did a good temporary fix on a head gasket problem but clogged the bottom of the radiator after 3 applications over 12 months. Too much gunk. After a rodding and rebuilding the radiator - no more temperature problem. You may have purchased a truck with a similar treatment already done. I recommend you have your radiator checked, flushed and rodded. (Small Bucks) If you don't, you run the risk of overheating and blowing your head gasket. (Big Bucks)