That is similar to what happens up hills with the LR cruise control. The car slows down, probably on full throttle, until it decides to change gear and then speeds back up to the set speed. Sounds to me like it was on full throttle into the wind and not quite at the point where it changes down so it never sped up. If you manually dropped a gear it would probably have been able to hold 90. Mine drops to mid 80's before changing down a gear. Even the D5 is the same, but it stays a bit closer to the set speed than the D4.
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