Corey,
All those reading are good/typical and the SG is likely working fine. I would look for grounding issues and bad connections first, look at the voltage regulator ground. The voltage regulator operation can be intermittent too if it is old and corroded inside or an aftermarket POS. It is had to diagnose a problem when it comes and goes. Next time it is not charging take voltage readings to verify.
My 149 had a charge indicator that was goofy. The needle would bounce side to side and that act ok. I finally determined it was defective and pulled it apart, found the small magnet inside has only partially attached to the surface of the gage. I glued the magnet back in place so the indictor needle rested in the center, reassembled and it has worked fine ever since. This meter was not an IH original but some AMPOS.
jim