Keyswitch and supply fuse, check input voltage to primary side of coil. if voltage is low trace it back to the poor connection. I had this happen wehere someone had replaced the fusible link on a truck with a 30A fuse and cheap holder, and corrosion on the output blade melted the fuse and holder so it only fed the coil intermittently.
Wiring is where I would have started, not ended... a distributor could have bought and paid for a nice FLUKE meter...