Also might be a bad engine/firewall ground wire or bad ground on ignition box. In addition to Terry's advice, check the distributor cap for a carbon track. These often develop along cracks in cap, shorting spark impulses to ground or to another plug wire terminal.

A bad distributor vacuum advance cannister can cause some serious problems, especially bottom end power. While you have distributor cap off remove the vacuum tube going to vacuum source and apply vacuum to it while watching to ses if advance plate moves/turns. Hold that vacuum for a moment to make sure it doesn't bleed off through a ruptured diphragm.