Rough Running at Highway Speeds

[BR3W CITY]

Disconnect your MAF and tell us if anything changes.
My perennial question, who did the tune? It sounds like your issue is only occurring at a specific Kpa (cruising at med speed, low throttle, low load). When you goose it, your forcing past the flat spot and it picks back up.

Vacuum leaks, not enough timing can both have some of these effects.