Charging intermittent, sometimes it does sometimes not

If the 12v is happening at slow idle, IE at a light with foot on the brake, this would be normal. But voltage should come right back up to "nominal" 14V as RPM is increased and you start to drive

This sounds intermittent

Check field circuit, especially look for bad brushes, loose connections. Check voltage supply to VR IGN terminal.

Check charging path (output) Monitor output right at the alternator output stud and see if it goes down with battery voltage, or whether it "spikes up" at some point and battery dies down. This would indicate an intermittent open/ bad connection in the charge wire path, wire end terminals, bulkhead connector, ammeter, etc