Holley problems

I have a holley 780 vacuum secondary carb that causes the engine to break up at high RPM (I assume from loss of fuel). I have gone throught the carb and cleaned everything, replaced the power valve, needle valves, floats, gaskets, secondary diaphram, etc. and it still does it. The car runs great for general driving, just breaks up when I get on it.

I know it is in the carb, because if I replace it with another carb of the same model, the engine runs fantastic at RPM. I don't want to just use the carb that RPM's well because it is very old with a worn out throttle plate, thus causing the car to not run well in traffic, where I tend to use it the most.

Any ideas what could be wrong?