2000 ford explorer ohv push rod. Hasn't really been running right to begin with. Could barely make it up to 55 mph Replaced plugs(drivers side engine bank. Spark plug gaps where over .1 inches) Replaced wires Crossed a wire(caught the cat on fire) Got the wires set correctly Welded a piece of exhaust back on(battery removed) Replaced fuel filter Where I'm at now. Starts like a new car. Engine revs smoothly through rpm range. Put the vehicle in either reverse or drive. The vehicle does move under it's own power on flat level ground. Any engine load and the engine starts stumbling ultimately stalling the engine. Thoughts?