newbi question

hi all, i am new to the forum and would like to stay as a member as long as i can. i have a 1992 2/door 4x4 Yukon with 170,000 on it and still running super strong. i also have a 1984 s10 one owner with 14,000 original miles on it that i am swapping a 350 vortec roller motor on it this winter. but back to my question is. on my Yukon, I'm having a problem with it charging properly. i look at the gauge and it bounces 14-11 volts. one min its at 13-14 volts and the next its at like 12-13 volts. it does this slowly. alternator was replaced but still the same. i do see that the plug on the side of the alternator has one skinny wire and the other one is for the charge. i have a stereo system in it and my subs amps kick out when the bass hits a little hard and i think its when the charge drops to 11 volts. any idea's on this issue. thank you.