I had a very similar symptom for several years on a Citroen SM (Masserati engine). The problem was finally solved when I disassembled the solenoid and found the contact bar was bent slightly and thus not making good contact when the solenoid 'pulled in'. The starter would bench check perfect, then work erratically installed. I suspect a defective starter solenoid is the root cause of the erratic start. A bad commutator segment is very unlikely.