Welcome to the Forum.
I had a problem with my 85 300D suddenly pegging all the way Hot. sometimes the needle would fluctuate. When I turned the engine off and restarted, the gauge would read normal. I changed the temp sensor, changed the gauge etc.... took me a while.
My problem was the ground strap from the block to the frame. most of the one`s I have seen are bolted to the Adapter plate to the Body on the drivers side under the engine.
Charlie