Could be most anything with that description. Wiring harness, grounds, charging system, etc. Spend a little time cleaning connections and then check things with a voltmeter and you'll find it.

I had a buddy who had a Mustang with flickering lights. He had his alternator tested and replaced. He replaced the voltage regulator, charged the battery, replaced battery cables, replaced the headlight switch, etc. Finally replaced the headlight wiring harness and that solved the problems. Evidently the factory wiring harness had broken just enough wire strands that the lights flickered when he drove. Just about drove him batty looking for that flicker.