Q: Running Hot on 2001 Dodge Ram 1500

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Once my truck gets to its ordinary operating temperature it begins to run hot. Once the temperature maxes out, all of a sudden I hear water grugling in the dashboard then the temperature suddenly falls. Whenever the temperature is high the heater stops working. Once the temp falls the heater begins to work again. It does this until I begin moving at a constant speed but once I stop the temp goes back up, I hear the gurgling, then it falls.
(1) Answer
Really need to inspect your cooling system. How long since you replaced your thermostat, which should be done every 60k. I would do a complete flow test and pressure test of the cooling system and verify that your t-stat is opening at the proper temp. I would also do a chemical block check to ensure that your head gaskets are still ok.