Wow, I thought I was the high miler at 270k!

Just a hunch but whatever is causing this is probably always there but only noticeable at a consistent cruising speed.
Around town your speed is not as consistent,rpm's are up and down so if you were to set your cruise control at 25mph it might appear?
Not a solution but may add to helping diag the problem.

LIFT OR BUST! 2001 Sportage EX 4dr 4WD with 300,210 miles & counting .....
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