Sometimes when I drive my 2013 Civic Si sedan, the headlights will randomly dim then get brighter, vacillating between bright and dim. I believe it happens to both headlights simultaneously. I don't think it's related to hitting bumps in the road. It happens only when the car has been sitting out in the cold for a while. After I drive for a few minutes, everything is fine. I haven't noticed any other electrical problems or flickering on the dash - all the indicators are steady. What could it be??