Watch the "Volts" gauge on the dash. If it flickers at the same time, and only when the engine is running, suspect the charging system. If it stays steady, check the head light switch connector and terminals for signs of overheating. Those switches have thermal circuit breakers built in too that can develop pitted contacts that make intermittent connection. You may also see the interior lights do not flicker, but if they do, it still could be related to the switch.