Why does my car have no acceleration over 3000 rpms?

I have a 2001 saturn SL. 2 weeks ago when I was driving, I tried accelerating and it was very slow, as if it had no power at all. It would never go over 3000 rpms. Everyonce in a while it feels like the engine shakes really bad and tries to accelerate normally but for only a second, then it goes back to sucking. I just changed the spark plugs because i had a similar problem with another vehicle and it was because of bad spark plugs. Even though the spark plugs needed changing, it did not help this time around. I was told it might be bad gas but i have refilled it since then and with fuel injector cleaner and its still running bad. Any ideas. I'm too poor to take it in to see what it is if its something i can do.