You said new heads.....which springs did they come with and are they a match for the cam?With fuel sometimes it will happen at different rpms,depending on how much got back in the bowls between hard runs.
Spark and springs seem to act up at the same rpms ,over and over.
Whats the exact sound it makes???????Does it seem like its bogging,making tons of noise with the carb ,but no go,a good steady tone when it bogs?
Or does it make a gargle sound,the gargle sound is valves ,the big one tone bog is fuel and a highly variable surging bog is usually spark related.
Does it make the same engine sound when it does this every time?