I'm experiencing a strange situation with my temp gauge on my 85 300D....When I first start it up, the gauge shows cold (below the 40*C mark) and I've watched it quickly climb to about 100*C (as expected) over the next few minutes. It hovers around 100*C for a while then it pegs to the top of the gauge (like as if it's grounded?)........At this point, I can shut the car off and put my hand on the radiator, the engine is just starting to feel warm, it's nowhere near 100*C.....At first, I thought it must be a bad gauge, but I see the same symptoms after switching it out with another one (both could possibly be bad I guess) but it seems like something is causing this "grounded" condition as soon as it starts to warm up....Could it be a bad idle temp sensor (on the thermo housing)....??....any ideas?

I'm sure one of you experts here knows what's going on and how I can resolve this issue.......THANX for your help guys!