Yo man, it is a very common problem. i am some what going through the same thing. What you most likely need to do is get a new IMRC (Intake Manifold runner control) box. it controls your secondaries, which is what gives you most of your power after 3500RPM. This most definitely sounds like the source of your problems.
An easy way to check is start the car, pop the hod, manually push the throttle in and see if the little connector to your secondaries moves once your RPM's rise. If it doesnt, you need a new IMRC, or you need a serious cleaning of your UIM and LIM (it may be stuck on gunk).
Hope this info helps you.
