After the cap and rotor, gently dry the plug wires. Water on cracked wires also causes problems.
If it might be the plug wires, you can diagnose it if you've got a shaded area where you can open the hood.
Warm it up to full operating temp. Spray a light mist of water on the plug wires. If the wires are at fault, you might see sparks jumping to the valve cover, or some other grounded metal.
If you do see sparks, my advice is do NOT touch the wire with the engine running.

Guess how I know that's a bad idea.