Why does my temp gauge go down while driving?

SOURCE: engine overheat

Hi and welcome to FixYa,

Offhand it would appear that the radiator fan is not working. In some versions, either the A/C or the heater must be turned on for the radiator fan to work. In some other incidents, the fan utilizes a clutch. Common design makes use of the inverse property of silica oil to serve as the clutch. Still others have a relay that actually does the switching on of the fan. Of course it would be to your advantage to likewise check the temperature sensor on the radiator.

Good luck and Thank you for using FixYa. Happy Holidays.

SOURCE: 2001 Dodge Durango 5.9L R/T.

Change the fuel filter. An put some injecter cleaner in it. With some good gas.Then hit thr highway an blow it out.Not to fast. lol

SOURCE: engine overheats when heater pipes connected

I think you could be right about there being air trapped in the system.
Or the heater core(radiator) is plugged.
Either way you need a pressurized radiator coolant pump to put the coolant in without air being trapped.
Although,i have seen it done by jacking the vehicle up high enough that the radiator was now higher than the heater core level, and then the coolant was put in.
But this is dangerous to try.

SOURCE: Overheating/No Heat 2001 Impala

change the thermostat and the temp gauge in the radiator

SOURCE: VW Jetta 00 water temp gauge

Sounds like water pump or your thermostat the second is cheaper