When you get cap and leads make sure you get a rotor button aswell. I had same problem, it was a dead lead and a cracked rotor button.
One usually follows the other. If your spark plug gaps were too big or your lead has too much resistance. Electricity is lazy and will always take the path of less resistance. This is why it will arch across the button or cap and generally under load (driving up a hill).
Look very carefully for cracks in the button and cap. It doesnt even need to be a crack, it can be a line of carbon deposits (burn mark).
Good luck