Was the car a "city car" before this long trip? If so, all of that driving at 75 mph probably got to some of the carbon in the engine and "rearranged" it., making it run rough now. I'd try some more highway driving and see if that helps clear the rest of the carbon out.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.