2002 Sienna - all lights flickering

All lights (headlights, taillights, inside dome light, dash lights) are flickering on my 2002 Sienna. I've already had the alternator replaced, and the lights are still flickering. A second new alternator was even tried, and still all of the lights are flickering. It's just a slight flicker usually, but sometimes (especially at night), it is very noticeable.....like a strobe light! Any similar experiences, advice is GREATLY appreciated!