Can you tell how high it goes? One think you might consider is temporarily installing a voltmeter so you can monitor battery charging voltage. The problem is "where to hook it?" If you hook it to say, the ignition switch/ fuse panel, and it's the typical harness--to--regulator voltage drop problem, it will show "normal", please don't cause me to explain that.
If you hook it to the battery, you will need a separate switch or relay, but that WILL show the "true" battery charge voltage.
Might be as simple as a loose connection at the brushes/ VR connector
Might be one field/ brush intermittently grounding, or even an intermittent short in the field winding (rotor) itself
I admit, intermittents are tough.
