This might not be your problem but a friend of mine had the same problem of popping through the carb, it was under the same conditions you describe. He also has the edelbrock set up with cam, lifters, intake and the performer RPM heads.
He changed the carb, jets ignition, plugs, checked intake gaskets for leaks, checked and rechecked timing. Each time he thought he had it fixed and then it would pop through the carb again. We did notice that it seemed to be worse when it was warmed up. We even did a compression test on it and it was ok, also reset the valves, it still popped.
One thing I did notice when setting the valves was that #2 exhaust valve rocker wasn't getting oil up to the rocker. I figured a lifter problem.
It took alot of work but finally talked my friend into changing the lifters. It no longer pops, it seems that that #2 exhaust lifter was colasping and causing the popping because the exhaust wasn't getting out of #2 cylinder very well. If you happen to have the same problem it makes me wonder if Edelbrock sent out a bad batch of lifters?
One other thing that will cause an engine to pop out of the carb is not enough timing, make sure you have your timing right also.