I have seen engines exhibit this hot/cool action with a head gasket leak because it bubbles the coolant out of the head.
But this thing has no other head gasket symptoms. No extra pressurizing, doesn't blow any bubbles, no exhaust or fuel smell in the tank, no coolant smell out the tailpipe & the water level doesn't drop over night.

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85 300SD. 88 300SE. 85 560SL.