98 5.4 coolant level issue!

[Calabrio]

I'm inclined to think it was air in the system.

How long had it been doing that?
Was there any repair or service that had taken place just prior to it?
I think these two questions are critical to the internet diagnosis. If the system had never been opened up and it just started doing that, the line of questioning will be different than if you say it started after you changed that PITA lower radiator hose.

Does the heat in the back work?

 

[rjdelp7]

I had similar issue on a Lincoln town car 4.6l. There was air in the crossover tube. The coolant gauge was reading low and fluctuating. It had a weak heater. I removed a plug, near the thermostat and added coolant directly. It took about a 1/2 a quart. Right away, the gauge went to mid range(warm) and reservoir was stable. The heater worked correctly and was very hot.