driving? Seems to pulse more when accelerating, and when the gas pedal is released the lights stop pulsing, but then seem to dim and sometimes come back to normal. Usually start pulsing again though......
Check the battery cables, and the connections to the alternator. Belt tension is important too. If this doesn't fix it, have a qualified technician find the problem. Electrical malfunctions tend to be either extremely simple, or nearly impossible to diagnose and repair, especially intermittent ones. Good luck with it.
