fuel problem
try non alcohol gas, run the tank down fill it up a couple of times then test again.
I had same problem (not at high RPM but under load like starting from a light on the street, stock 273 2bbl) only when hot out side or idling for prolong periods of time (drive through lane) then relatively normal startup from a light.
I could watch the "phenomena" happen at idle. I had a clear filter, hot engine, filter fuel level would slowly drain down to nothing (pressure would stay at about 7 or so), engine rpm would start to drop (pressure would start to drop to 3ish) then pick up as the filter started to refill (pressure would return to 7 ish)
doesn't happen with non alcohol fuel!