I had similar problems with my sister's cougar. I spent a bunch of time replacing parts with little improvement but then as the car became worse i decide to pull of the intake manifold to clean the injectors and found more carbon buildup than i have ever seen on an intake.
I spent 2 days cleaning it and after i put it back together it has never run this good. The intake is easy to take off so i would give that a shot. I have pics but i can't seem to post them.
Mark