Hesitation and Shuddering with No power

[pheasant plucker]

I scanned my 02 GP GT , 190000km, and got P0122 TPS, P0137, P0141 Heated oxygen sensors.
I ran without the MAF plugged in and it didnt change.
Changed the tps, actually got a bit worse.
Cleaned the TB, no change.
Cleaned the IAC, no change.
Replaced the downstream O2 sensor, no change.
Changed the plugs, no change.
Changed the upstream O2 sensor and things went from bad to worse!!!
Checked for vacuum leak but pretty hard to find anything.

Now it seems as if the catalytic converter may be plugged???

This sounds like it will do the trick but what are the options for future header install? Should i get the 2.5 DP with Ubend delete? i cant say i am totally up on the jargon here but hopefully you guys can give me the lowdown!!

Meegwetch!!

[00grandprixgtp]

i would get some headers but then again it is just a gt so a down pipe would prob work as well ebay speed daddy knock offs work well and are only like 200 shipped

[pheasant plucker]

Just read here from siwittschulte http://www.grandprixforums.net/i-stu...elp-67371.html that he fixed his chugging problem with a ICM from the junkyard after he added a new cat??? can i replace the cat with a straight pipe?