Start my 1930 model A , sometimes not charging, not output from cutout to generator. Quick jump front to back on cutout, starts charging. It has semi-conductor cutout. Third brush set to approx. 8-10 amps.
Diode failure is rare, and would likely be total instead of intermittent. I would suspect worn brushes/weak springs; possibly ground connection of field windings.
I had a similar problem. I found that the nut and washer that holds the cutout on the generator output post had a layer of rust making poor connection. After cleaning the rust the generator is charging properly.