408 breaking up
Here's something else to throw in the mix after you've checked the other suggestions individually: you didn't say what type or cfm carb is on your engine.
Since it's a high-speed problem, and sounds like a lean condition, maybe you don't have enough carb. Years ago I ran into the same type of problem on a stout 340. The terminology of the day was "shooting ducks" as it was a rapid popping through the exhaust at high rpm's. The engine ended up needing a 850 double-pumper to keep it well fed and happy.
