I would check to make sure the total advance of the distributor is within specs. Make sure the vacuum advance (if used) is operating correctly. Carefully run all the plug wires to assure no issues there. Is the distributor in new condition with tightly bushed shaft? A high pressure oil pump can wreak havick with a worn distributor shaft making the timing act erratically.

The problem could also be fuel related. Install a fuel pressure gage at the carb and prove to yourself that it is seeing 6 psi throughout the speed range. If you are running a carb with a power valve, is it the right power valve for your cam/vacuum conditions? Finally is the valve train within the engine fresh? Tired lifters/springs can bring down the onset of valve float and cause erratic operation. This should not be an issue with a properly rebuilt fresh engine.

Good Luck, Kitz