Well, I can only provide observations from my '87 300D.

It has run consistently right on 80C during all the cold/cool/lukewarm weather. The past couple of days outside temperatures have been in the 90s here, yesterday 95F. In stop-and-go rush hour, the temperature crept up to maybe 83-85C at one point. The auxiliary fan would kick in, I presume because of some AC data point rather than a high temperature reason.

At some times moving along, the temperature gauge actually fell below 80C - can't figure that out.

I don't know if my thermostat is original or some aftermarket (lower temp spec) unit, but if the gauge is accurate then I'm pretty happy running at 80C.

As a complete guess on my part, I don't think MB temperature gauges are necessarily all that accurate. Yours sitting at 85C may actually be running the same temp as my car with my gauge showing 80C. But I'd say the relative change you are seeing under hot & AC conditions is something I'm not seeing.

If I were to make a wild guess - I'd say maybe its time for that citrus treatment in your cooling system that people discuss here. It sounds like there might be a slight limitation in your cooling capability.

Ken300D