When I pull a boat/trailer (4,000 lbs) out of the water on an inclined launch ramp, my 2005 Navigator occaisionally will not accelerate. The engine does not stall/turn-off, there is just no power to go forward. I usually have to put it in neutral and rev the engine, and after a couple of times doing this, it will finally go forward. I use premium fuel. Does anyone know what may be the problem?