Change the thermostat to a stock spec one, maybe on the low end of the allowable temperature scale.
Sounds like you've got too much cooling going on which isn't allowing the thermostat to fully open at highway speeds. I had this problem on an old Jeep Cherokee of mine after I put an air vent in the hood and added a big engine oil cooler. The engine wasn't getting up to temperature when driving so I had to get a thermostat with a lower temperature rating.

Be careful though, if you're not getting good flow of coolant through the engine, this can also restrict hot water to the thermostat and heater. You can get overheating pockets in the engine. It may benefit from a flush, then again, a citrus flush may cause leaks. It's always a gamble.

Good luck!

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson