1982 Evinrude 35 hp. Runs and idles fine at startup but, after it gets warm it loses a cylinder, top cylinder, at idle. When going to full throttle it will suddenly "take off" running on all cylinders until back to idle at which point it will lose the top cylinder again. I'm thinking spark issue, coil, power pack, not sure. New plugs and wires, good gas. Need help. Looking for sound advice not guesses.