Temp gauge issues on 1996 Ford Explorer

1996 Explorer Sport - while using the heat my temp gauge will shoot up to about 3/4 and then will drop all the way back down.

Have tried different thermostats and replaced the sending units. Tested hoses for leaks and there are none.

Can anyone let me know what else to try? I'm afraid to use my heat because I don't know if my car will over heat or not.

Asked by for the 1996 Ford Explorer