If it happens at almost exactly the same time every time you drive, when you reach highway speed, then I don't think it's air or debris. If it were, I would think that your loss of heat would be very random, and it would come and go, not just go and never come back.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C