Check the "Z" connector coming off of the driver board at J114. If you are unfamiliar with this "Z" connector, it is like a coupler/splitter used to split the power so they could drive the aux 8-driver board. If you can disconnect the plugs from the Z connector and the reconnect them tightly and the flipper works then, that is your problem. This is only a temporary fix. You should replace the connector at J114 and crimp the two wires together in the connector with Trifurcon pins and eliminate the "Z" . This fixed mine right up. I got this info from the Pinball Ninja site which I highly recommend.

I believe over the years the pins can become tarnished are just don't make a solid connection and also the two connectors that plug into the z and the one on the board are IDC connectors which can also be problematic causing voltage drops when a load is put on them. Like I said in my earlier post, reseating is not a permanent solution. It WILL happen again at some point unless you eliminate that z.