I once had exactly what you mentioned going on in a 454 powered jet boat. It ran great until I brought up the rpms a bit above idle or placed the engine under load then it would pop through the carb. A number of people told me to look for a flat lobe on the cam but I just couldn't beleive it since it ran so well at low rpms. I fiddled with the timing and carb and gave up and pulled the valve cover.
It took me two seconds to see while turning over the engine with the starter to see that the No.8 exhaust wasn't opening as far as the others. Pulled it apart and sure enough, a flattened lobe.
Since then I've found others with the same problem and all have discribed what you are dealing with.

Just a thought........