Yes definitely something else going on. possibilities are that the green wire from alt to reg is intermittently touching metal/grounding out & making it full field for a split second and related thing with the alt brushes/field circuit (them grounding out & full fielding it. got another alt you could toss in for a quick test? for the "green wire" possibility, a good visual along its length or pull the triangle connector from the flat reg & pull the green wire off of the alt & with a jumper connect alt to reg (side terminal) and jump 12V (blue ign1 circuit) to the reg top terminal. the recessed reg nubs are a slight pain to jump. EDIT I'm wondering if it might be the digital meter? if you have a 12V bulb such as a 1157 lit up nearby does it change brightness when the meter changes reading?