Code P0171, sometimes run pretty good and other times runs kind of rough?

The truck listed above is a Super Duty pickup that sometimes run pretty good and other times runs kind of rough. I have changed plugs, coils, PCV valve and air mass flow sensor on it. Check engine light on showing code p0171. Sometimes smell strong gas smell from exhaust. At around 55 MPH the truck engine runs smoothly just rough at lower RPMs. This is puzzling like I said. Sometimes the truck runs pretty smooth even at lower RPMs and other times runs rough what could be the problem?