[spoiler][/spoiler]Sounds like it might be the distributor trying to add timing at cruise RPM
I try to recurve all my distributors to get all the mechanical advance in by 1600 to 2000 RPM max
Have you tried tuning the outboard carb idle fuel ratio with the idle fuel adjuster screws in the base plates? If not try gently closing them until fully seated and then back out from 1/3 to 1/2 turn out from fully, seated depending on what altitude you're at
Six packs rule when they are tuned properly tuned : scope