I have a 1997 Grand Touring 500 that starts great when cold and will idle forever. But when driven it goes so far and just quits like you turned off the key. We pulled the plugs and no spark. After it cooled off we put in new plugs and it started right up. It then ran great until again running it a ways (about the same distance as the first time) and it just quit again. Again no spark. Any ideas where to start?