My thought also was brushes. When worn almost at the end of their life they can sit a bit crooked in their holders & not make proper contact with the slip ring. This may explain why seeing the lights is not consistent.
@
Growler - do some alternators not have replaceable brushes? Haven’t come across one before?
Another possibility whilst I think of it, only ‘cos I had it happen to me once - brushes were fine but the plastic brush holders had hardened, cracked & were in the process of braking up - same symptoms.