[bobcowan]

Plug wires are a replacement item, just like the plugs. When they start to go, they leak electricity through the insulation. When they get really bad, you can see a blue haze near the wires when the engine is running in the dark.

A good set of plug wires is pretty cheap, around $100. If they're old, I can't think of a good reason NOT to replace them. One my race car, I replace them every 2-3 seasons just as preventative maintenance.