I have a 1999 ford explorer, 4.0 v-6 sohc, a/t. The enginge bucks/lurches when a load is on it, like going up a hill. This happens between 50-60mph. If I kick the pedal to the floor, and release, it seems fine. I have replaced the i.A, c. Valve, spark plugs, plug wires, air filter, fuel filter, throttle control, upper and lower intake gaskets, and the m.A. F. Sensor. No c.E. L. Lights come on. What can I try next to fix this annoying problem? Thanks, and have a great day!