I have a ford 2.3 in a dirt track car.  It has a comp cam, double springs, anti pump lifters, adjustable cam gears, holley 500 carb, carter marine aftermarket electric fuel pump, accell hi voltage coil. stock points distributor, race gas used. 
Here is the problem......t he motor idles fine.  when you take it up to about half throttle and hold it there it develops a bad miss.  same things happens in gear.  can idle or rev high and its smooth.  Hold the throttle and it misses in gear or out of gear.
what I have tried....new plugs, new wires, adjusted distributor for timing issues, no vacuum leaks that I can tell, new gas, tried 2 different Holley 500 and did the same thing.
Any ideas on what could be the problem?

Hold at half throttle and see if fuel is siphoning out of the accelerator pump squirter. Had this problem before with the 500. If so, a heavier check valve under the pump squirter will help. Holley makes an anti=pullover squirter also. I had luck using long tube squirts with the heaviest check I had...had a stainless one from an alcohol kit. 

Float adjustment?