Faulty coils will do exactly the thing you're describing. Just run the thing up to operating temp, wait for it to punk out, then shut it down and use a can of freeze spray to cool the coil gradually. Don't just hose the thing down because you'll ruin a good coil that way. Also don't hit any cast metal with the spray because cast metal makes funny noises when shattering. A spritz on the coil here, a spritz on the coil there, a few seconds apart, and keep on until the coil is cool to the touch. If it then fires up and runs, bingo bango bongo.