It could be spark plug temperature rating. Try going to a hotter plug (but not to the maximum) especially if you have a lot of miles on it and therefor maybe a wee bit lower compression value. When you come off the highway you are at a different temperature soak then 20 minutes later your motor temp soak has equilabrated to a stop and go traffic value. This is just a hunch but easy to try! Vacuum leaks are really likely but you've been through that, right? ;)
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Genesis 7:11