My 2004 Ford Ranger with a 4.0l v6 and a 5-speed automatic transmission would shudder when I went from no throttle to moderate throttle. It almost felt like a rough shift, but it wasn't during gear changes. I added STP oil treatment and a gas treatment, and now it revs up smoothly. Do you have any idea what the cause of the problem was? Should I be worried about it, or just continue adding STP?