Ignition issue, paging Del

First thing I'd do is check the belt, second thing is check brushes.

If neither, then "try to get it to act up" (We ALL hate intermittents LOL) while monitoring one of the other field connections. The blue wire field should always have 12V in "run" and the green will vary, but should be "low" when charging. That wire acts as a ground controlled by the VR. Other than that check the ground on VR, and work the VR plug in/ out several times in case it's corroded.