Once My RPM goes over 2000 my truck stops accelerating but the rpm still goes up (as if it were in neutral). I had the problem once about two weeks ago but after i turned the truck off and turned it back on, it ran normally for about a week, and now its a constant problem. Its a Ford F-150 year '99. I changed the transmission fluid with no luck. It runs fine under 2000rpm. it has over 160,000 Miles on it. The Check Engine light is on but it turned on before the problem started. Let me know if you need more info. I figure its probably hard to help on the Internet though.