I would go ahead and replace cap and rotor, unless they are close to brand new; they are not terribly expensive. Just recently a friend pulled his rotor while looking into a no-start and although it looked fine externally, internally it had arced and would not pass current. The cap, too, may have a problem with the little carbon button in the center.

The other place to look is the plug wires. Replace them if they're more than a couple of years old or have 50K on them.

And in case your parts are all close to brand new - is the coil wire fully seated in the cap and on the coil?

good luck, and let us know when you find the problem.