My car has seemed to lose HP since I bought it. Possibilities? Details inside.?

I have a 97' cavalier 2.2. Since I bought it about 18 months ago, I've noticed it's lost some juice. I realize it's no speed demon at all, but I used to be able to feel like the car was actually going somewhere in the beginning. It can really seem to accelerate the same from 40+, but getting to the 40 seems like forever. To me, it feels like the engine just isn't going to even 75% of what it could. I'm thinking possible dirty fuel injectors, but I'm sure that would be too easy and wouldn't effect it THAT much. Any recommendations? Should I just go to a local shop and get a tune up and get everything inspected? It has 120k miles, so I'm considering selling it while it still has some life and value in it.

And I actually just replaced the air filter about 3 months ago and just checked it 2 days ago and barely had a spot on it, surprisingly.