ammeter is bouncing

i put a regulator in the 74 duster today. it was doing some funky things. sometimes not charging, sometimes over charging. most of the time it was fine. today when i was troubleshooting it, i saw it go as high as 16 volts. i had a new napa regulator so grounded it right to the alternator and plugged it in, sure enough the 1974 mopar regulator was acting up.