Temp Guage Issue
I have a 99 V6 4.0 Ranger and a couple days ago my Temp Guage started acting up. I would start my truck and it would warm up to normal (Mid temp) and then my heater in the car would go cold, then I would see my engine temp gauge go down to cold, then go almost to the red, then back to Normal. After this the heat would go back on.and everything would be fine for the rest of the ride. I have just replaced my thermostat and this morning it did it again. This happens everytime I first turn on my engine, from sitting for a while.
Any suggestions on what is wrong?