the Tstat sounds. . .
like it's a good place to start. Sounds like it's stuck partially open.
Also check your af/w ratio: I firmly believe in a 40/60% ratio (af/w) and use Mercedes AF and distilled water. If it's 'dirty' this might be a good time to thoroughly flush it and refill with fresh AF and distilled water.
One of my Cool Harness will help also after the above is done.
Regards . . . . JimF
'94 S500 Cpe
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