I have a 99 f-150 that has been stumbling and hesitating for a few years now.It seems to be a common problem so I replaced the same things as everyone else(fuel filter, plug wires,plugs).When I replace the fuel filter. it is better for a couple days and then goes back to normal. You can put the pedal to the floor and there's very little or no acceleration.In the winter, i HAVE TO PUMP THE GAS to get the gas flowing and it still struggles to start. If I add gas line antifreeze and put in premium gas it starts a little better but its too expensive for each cold night! Anyone with similar problems?