How to troubleshoot varying alternator voltage?

[richard123vmt]

This is a 71 vw bus with alternator conversion. It is a bosch with internal regulator. I have not run it much in the past few years. Anyway, I have a voltage meter on the dash. The alternator light comes on with ignition on and goes out as it always did upon revving the engine. The light never comes back on.
But while some of the time the voltage reads about 13-14 volts (again, the long accustomed position) at other times it is only about 12. In other words the alternator is not charging or charging but feebly.
I observed on one occasion that hitting some bumpy ground causes the voltage to rise to normal. This made me think I had a bad connection in the front near the fuse box. But no amount of moving the wires has produced the symptom.
Today, I followed a different tack, which was the possibility the alternator rings had gotten funky due to sitting several years (I did drive it occasionally). So I goit the brush holder out part way and blasted the interior with PCB Blaster (had no carb or electrical cleaner). After this only the lower voltage. However, I later took it out for a run and on return 13-14 volts. There is a slim chance I cured it and that the PCB Blaster film temporarily impeded the charging. However, based on past experience with the intermittent nature of this fault, I think I need a full systematic diagnosis. I have read the charging voltage when high to be 13.6. Maybe I should check at the alternator & battery when it is showing low. Then if it reads 13.6 I will know it is the wiring to the volt meter. (I have substituted meter itself). Any insights?
UPDATE: I am inspired with a plan. If and when the voltage reads low, go to the back of the bus and measure the voltage. If it is in fact low, then run a jumper from battery + to the regulator terminal. If the voltage does not rise, then the fault is internal to the alternator. Or so I think. But of course, I am betting against myself. Maybe PCB Blaster came through for me again! I may never see the low voltage again.