I had similar problems with my wifes 300SE. Was chasing it down, including changing Monovalve with no improvement, when the water pump went south. Since replacing the pump heat has been fine. I'm suspicous given the number of miles on yours that you might be heading the same way.
Hope this helps.
Steve

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Steve Short
'91 300SE sold
'92 190E 2.3 sold
'99 S420
'06 Mini Cooper S
'08 Triumph Rocket III
'02 CLK430