All the other engine build parameters are irrelevant. Even if it were the 6AL box, and when the RPM's hit 5100, 5200, 5300 etc, that break up would definitely be noticeable and consistent.
It is a fuel starvation issue. I am 23% sure of it.
Si Vis Pacem Parabellum
1967 Starlight black PMD Engineering 400 Auto 1968 Alpine Blue 400 4 speed 1968 Verdoro Green 400 HO 4 speed 2013 1LE 2SS/RS Inferno Orange Camaro.