I had a friend in High School that had a '69 Fury Convertible. Very rusty car. If you turned the hazard flashers on, the two-segment tail lights looked like wig-wags. That's where I first learned about ground issues

denfireguy raises an excellent point. Could be faulty grounds at the socket, too. Make sure everything is clean. Also double check to make sure your bulbs are the correct type. Cramming a single-pole lamp into a double-pole socket can also create weirdness.