The power wire to the distributor should have close to the same voltage as the charging system puts out when the engine is running. Low voltage can cause misfires. Look for bad connections or a bad ignition switch. The distributor mounting pad must be clean of paint and corrosion. Contact between the distributor and the engine block is crucial for current flow through the ignition system.
Bad contact can cause misfires.
Finally, check the distributor advance weights beneath the rotor for free movement. If they are stuck in the advanced position, rough idle and misfires can occur even though the ignition timing has been set.