Any One Heard of this....

I have a 2002 Explorer Sport with just under 140,000 miles on it. About 3 years ago it started chugging. very rarely at first then gradually got o the point when accelerating over 40 miles to get on to a high or pass another vehicle it would constantly chug while I was pressing the gas petal. I brought it to a few shops they threw expensive fixes at me. one being a fuel pump replacement. I started little with a fuel filter. no change. Then did plugs and wires. The problem disappeared!! that lasted about 7 months now it's starting to chug again and gradually getting worse. What is causing this and how do I fix it?
Thank You!!