Check for brake drag, tire pressure lift each wheel off the ground and see if a tire has a drag, elimination of all ideas here, whens the last time you changed your fuel filter?
Also your pump could be going out, loosing pressure, or you could have a fuel leak somewhere and be loosing pressure.
Or the timing may me near off, bearings dragging, uhm, bad gas, vacuum leak, at manifold to head, or bad piston
