Some quick thoughts :O)
How does the ammeter behave when the blackout occurs? Going to zero = fault (open circuit) after ammeter, going heavily in drain = short somewhere.
When power ok, wriggle and bend cables, switches etc all around the bike and see if you can forcefully trigger the fault, to locate to some specific area to investigate further.
Drive with a multimeter (V) connected to the battery and see if the blackout comes with a sudden voltage drop (= probably some short to frame somewhere, or some serious internal battery failure)
The elimination method and see if the problem stops or continues:
- disconnect A from the regulator (with a fully charged battery, drive for a while)
- short the ammeter
- connect extra cable from battery to ammeter
etc etc, experiment (following the wiring diagram) until the culprit is identified.