Phil;

In general terms, variation in the light can only be cause by a variation in the voltage they are powered by...and a Generator, with its Commutator and Brushes and Relay VReg can certainly be making those for any number of reasons, and this is somewhat normal...BUT... since there is a HUGE chemical battery connected to the system power buss, in which chemistry and state of charge is responsible for the voltage, and this simply cannot instantly change, I'd say that either the Batt is not well connected OR has a open cell...so due to this, the power system has lost the ability to filter out variations in voltage, and these are then able to be seen as the variations in headlight output...sorry for the long explanation, but this was not a yes or no question!

Bottom line...I'd measure the sys V RIGHT ACROSS BATT Terminals (NOT Clamps), and if it does NOT vary (dip) synchronized with the a dip in the headlights, I suspect a poor connection between Gen output (VReg D+) and Batt (make ALL fat cable connections shiny metal, snug, and protected with ACZP (See: https://www.sw-em.com/anti_corrosive_paste.htm ), OR...if Voltage does vary, Batt has developed an internal problem (likely an open)...perform Batt tests! OR...if older than 4 years, simply replace it...

Good Hunting...let us know what you find, and solution!