if you turn on your lights, especially using a 90/100watts rating lamp, your alternator load will definitely increase due to high electrical current needed to supply your current load... one solution is have your lights back to stock 55/65watts rating or have your idling increased to compensate the alternator load when you turn on your headlights..

your alternator does'nt have the idle-up function just like to your airconditioning system wherein the idling is automatically increased when you turn on the aircon...

when you have high electrical current load just like having an over rated lamps, plus the load of your power steering pump when you turn the steering, your normal idling speed will be affected since the alternator and power steering does not have the idle up function...