GT Mustang V8 automatic, 67,000 miles, used as daily (only) car. A few months ago when starting car, it was hard to start then backfired, which was unusual, hasn't done it again. However, today engine didn't want to turn over at first when starting it up, started okay second try. It began feeling as though it was "missing" but isn't time yet for tune-up. I autocross car on short courses about once-twice a month from Mar to Sep. When driving from slow speeds and up, I feel it's running rough, a type of hesitation like possible misfiring, and have to push it slightly when accelerating, as if it doesn't want to accelerate easily. Could it sound like it needs a new oxygen sensor or a tune-up, new spark plugs? I didn't think I'd need a tune-up yet with this mileage, but try to drive hard when autocrossing it (35-55 second runs, 6 per event day--so not excessive). No engine light is on yet--though wondering if I need to take it in now for diagnostics and/or tune-up to avoid a break-down. Or if the engine light isn't on, is it something only minor? Gas consumption seems higher but being that it is now summer & am using air conditioner, not sure if it's only that.