2001 Cavalier falls flat on its face

When accelerating this car does not have any power whatsoever. I have replaced fuel filter and fuel pump relay switch(which is what a mechanic said it was after running a diagnostic test on it). While going down the road it feels as if the transmission is slipping to me. It seems to work fine until you give it to much gas then it falls flat on its face, but it also does this while idling in park and pressing down on the accelerator. Is there a sensor of some sort that could be bad or a fuel problem somwhere. Any and all help is appreciated.

When accelerating this car does not have any power whatsoever. I have replaced fuel filter and fuel pump relay switch(which is what a mechanic said it was after running a diagnostic test on it). While going down the road it feels as if the transmission is slipping to me. It seems to work fine until you give it to much gas then it falls flat on its face, but it also does this while idling in park and pressing down on the accelerator. Is there a sensor of some sort that could be bad or a fuel problem somwhere. Any and all help is appreciated.
Possibly a clogged cat. Go shake the cat around a bit, see what it sounds like.