Have you measured the voltage with a meter to see if its actually jumping around? It could be as simple as a bad bulb or a loose socket connection up in the headlight, but does the taillight do it to? If its both lights and the actual voltage is fluctuating as read on a meter then i would suspect your rectifier/regulator is failing or there is a problem down in the stator.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....