You can do the cap and rotor if you want, but they way i think is that if part of it is rigged, you dond know what else is.
Jegs sell a brand new ready to run hei for $79.99. You can find them on ebay even cheaper. Just get a new dizzy. Even if for some reason it doesnt solve the problem it sounds like yours is messed up anyway and i would replace it.
BUT, before you do anything, move the plugs around, you may have a bad plug on that cylinder. Or you can put an old one in it. Whatever you want.