You are on the right track. Vacuum leaks can be an issue when they aren't obvious, everything from a stuck PCV to a failing brake booster can throw it off. Have even seen leaks from incorrect valve cover bolts protruding into the intake ports and causing a leak. What are your MAP readings at idle and cruise? Hook up a vacuum gauge and see if the numbers match. There's a lot of charts to correlate inches of vacuum to kPa. You can also try setting it to open loop so there's no correction if you have a failing O2 sensor, it runs off the base fuel map when you do that. It's difficult to diagnose over the internet, so knowing what isn't an issue is going to help narrow things down.
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