bad engine miss?

ok so i have been experiencing a bad miss that just started for no particular reason. It happens whenever the engine is under a heavy load, normally around 3700-4200 rpms. if i take off from a dead stop it wont miss in first, it willl have a slight miss in second, and a pretty severe miss in third until it breaks out of that rpm range then its fine. I have brand new plugs, good gas, the wires are only a year old and look brand new, distributor cap looks good, no oil inside. its a 440 with a 750 holley. and the plugs arent fouled out either. any thoughts on what why this is happening ?