Temprature gauge
 

Please can you help me, I have a 1982 300sd ,the car runs great
but the temp gauge often shows a very high running temp.
When I put my defroster on for 30 seconds and hot air comes out onto the windshield, the temp drops by 1/3 inch down to a more normal range.

Is my thermostat out of order , is it normal for this car to run hot?
Any assistance is truly appreciated as I am a new member