why am i losing power?

i own a 93 ser with about 158000 mi on it. about 6 months ago i started noticing i had to down shift to forth, and sometimes third, just to get up a hill that i never needed to down shift before. i replaced my fuel filter and things were better. but now it is worse than ever, more down shifting and plus there seems to be no power band, no chirping in second and it feels like the car is working too hard and not getting the power from it. it has also become slow reving and untill about two grand there is a kind of puttering from the exhaust when you stomp it, even in neutral. what the hell is going on with my car?