I have a 2010 Fatboy lo. Bike had just over 20,000 miles. Lately when riding at night I have noticed my headlight will dim. Then it will get brighter again in 4 different brightness stages. Then it will dim again. This all happens in about 10 second intervals and just keeps repeating. I have replaced the bulb (cheapest option) and that didn't change anything. Am I looking at an altanator? Maybe voltage regulator?

 

Could be the regulator. Check the voltage at varying different rpm levels