Temperature guage or overheating?
 

I have a 1992 500 Sl. The temp will go from 90 to 110 and then fall like a rock back to 90. The fans are working . Any clues?

Tap it with a finger when high ( not you, the temp), if it falls it may just be a bad connection/ground/temp.
Do a search here, some specific info about that issue. I had the same problem, very erradic.
I had other dash board issues so I just replaced the entire cluster.