When I’m on the highway the temperature gauge on my 1994 Plymouth Voyager will go all the way to the H and then come back down immediately. When the car is parked and idling, the radiator will bubble and over flow immensely. It will over heat sometimes at city speeds, but not at often as it would on the highway. I have a brand new thermostat, and all of my hoses leading from my radiator have pressure and heat.
It’s only been doing this since I fixed the thermostat a week or so ago. The car has since gotten another new thermostat and has been flushed out multiple times. I changed the thermostat because my engine was running cold. We checked it and it looked to be the original.