Nissan Repair: 1997 Nissan pickup loses power on hills, oxygen sensors, muffler shop

Question
I have a 1997 Nissan pickup (85K 2.4L 5speed) that runs fine , until you start up a grade , especially with the air conditioner on . It seems as though it is starving for fuel, but once over the grade you can get it up to speed and it runs smoothly. Eventually , it will trigger the check engine light with a P0420 code. I have changed out the fuel filter , and checked the fuel pressure with a gauge and it falls into specs. I had a muffler shop check the cat , and he said it was fine , although the truck was idling at the time. Could the cat be partially clogged , and only affecting cruise speed operation? I also was wondering if it could be a faulty TPS .

Answer
Clint,

Replace cheaper parts first.

Replace the oxygen sensors first.  Nissan orignials can be obtained from courtesyparts.com

Disconnect the battery for five minutes to clear all codes.

If this does not correct the problem, and it probably won't, replace the converter too.  

I say this due to the fact that the oxygen sensors are long past due to be changed and the converter too.  I do the same thing, drive until the part fails.  And, the converter has probably done, as you say, and partially plugged.  But, that was due to coated oxygen sensors telling the computer they were running lean and that richened the fuel mixture to the point of overheating the converter until it partially melted...