One easy way to check would be to pull the starter, hook up an ohm meter across the leads, and slowly turn the drive gear. If there are dead spots, the meter will go from zero ohms to infinity and back again.
My guess would be either a dead spot in the starter, or maybe bad brushes. A rebuild kit will take care of the latter, but I think you may be looking at a replacement starter if it's the former.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.