I have a stock 1996 F3 that has started running real bad. It has a bad hesitation from idle, almost dies, yet runs well when I keep the rpm's up over 3000. I have changed the plugs and drained the gas. Could this be a rectifier going bad? Could this be from running racing gas in it for years, and now running 91 octane? Could it be a choke sticking? I don't know where to start - please help! Thanks.