check for play in your distributor, Wiggle it left to right. should move less then 1/8 inch.
Mine had the same issue and it ended up being the gears on the bottom of the distributor have worn and when I put the new cap on it was not lining up at the correct time.
If yo do have play you can test if it is the gears by taking the screws that hold the distributor off and twisting the whole distributor cap clockwise while running.