The flasher uses a bi-metallic strip, which could simply be worn out from flexing if it got a lot of use in the 50’s. When you say it sometimes doesn’t work at all, is that it never goes out or it never comes on? If it isn’t coming on, maybe the bulb is just loose, or you have a damaged bulb and the filament is sometimes making contact. The connections from the bottom to the top are the rods that hold it together and the friction connection between them and the clips could have tarnished and are affecting it. The voltage you use can also affect operation. Higher voltages will heat the strip more, keeping it off longer, lower voltages will allow it come back on quicker, but then the light is dimmer. You may have to play with that to find a happy spot.