I have a similar problem and I can't track it down either.

I'm also running about 100C now on the highway and I worry that it could be that my aux fans aren't working properly.

Occasionally, I have noticed that the passenger side fan won't be on or will come on intermittently.

I replaced the fuse after it stopped cooling all together one day and that seemed to help but it still underperforms in city driving.

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1993 W124 300D
-297K on the clock as I type this.