OK, now the car stutters somewhat at about 1500-2000 RPM (when the automatic transmission shifts). If I accelerate hard it usually backfires, and the Check Engine Light, which is usually on, flashes. When I use the "sport-shifting" feature it doesn't do any of this. My mechanic says it might be a sensor attached to the transmission. Any thoughts? Obviously, this might be completely separate from my other probem. Trying to avoid spending money but don't want my car to explode. Many thanks.