2002 monte probs

[Blackbird3800]

ok so my mother-in-law has a 2002 monte carlo 3.4 with 62k on it and it runs like crap. i had the light scaned and it was for an evap leak. well i remember her saying she lost her gas cap, so i checked and the one she now had on was a little loose feeling, so i bought a new one and reset the code and no more light. well 3 days later it still runs real bad. even after an air and fuel filter change. so this is what i can describe, in park or neutral it free revs smooth to 4k where its cut, but under load it has no power at all and holds all gears until you let off the gas. one mechanic said possibly a bad cat or fuel system pressure problem. i just dont know whats up with it, if you floor it from a stop it just bogs to like 1500 and very very slowly gains speed (slower that a bicycle), any ideas are greatly appreciated.

[Kain]

it sounds like the cat is getting clogged. my t-bird acted just like that when the cat was cloging. and evetualy stopped running all together.

[Ex-5.0]

Check plugs and wires first, then coils.