Try a new reg/brush assembly. Be aware that Bosch & the aftermarket make multiple variations in the set voltage of the regulator. If you think you need to nudge the output 'up' you can spec a higher voltage, ie:14v. This family of alternators was/is used in a lot of stationary & industrial applications. I have a client whose 85 w126 w/om617 did lots of idling & low speed w/AC on (hours & hours). The stock alt would not keep up. Switched to a 65/70amp unit, still had issues. Switched the voltage reg to a 14.2VDC unit, problem solved. Also replace or 'sister' (parallel) the main chassis to engine ground. They deteriorate after 30+ years of road salt, grime & water aided corrosion.