engine bogg

Ok, I have a 94 geo metro 3 cylinder.....it was running fine then it just started to loose power. Its an automatic so I had to manually put it in 2nd gear to keep rpms up. Now it sputters or sounds like a small backfire. But the car will start just fine, idle fine but when you try to go, it sputters and won't go over 45 or 50 MPH . I changed fuel filter, fuel pump, and still same thing. Any other ideas ?

Catalytic converter plugged possibly?

thanks for the help ... it turned out to be catalyst was clogged !! Got new one and all is well. Thanks again.:iceslolan