Needs fixing quickly

I have a 97 Mazda 626 4 cylinder with 94k on it. I have had it for 10 years its been good to me. Check engine light has been appearingP0421 warmup catalyst. I have been avoiding it because I know its the cat and thats pricey. I finally broke down and just fixed it because I couldnt find any newer car worth the money at all. I got it fixed at a local meineke new cat and I needed a new muffler. $678 and 24 hours later the light came on again with the same code?? So now what do I get the oxygen sensors fixed next and if so get them both fixed? My inspection sticker is due at the end of this month.

Also, a year ago I got a new water pump and timing belt. Ever since then my car has wanted to stall at a red light or stop sign. The idle goes way to the bottom and jumps around like its going to stall but never does. I had people look at it they dont see what the issue is. One thought the timing belt was off a tooth but that wasnt it. The caps and hoses are fine I have been dealing with it but would really rather it be fixed its simply annoying. Any ideas?

Sure sounds like a timing belt/cams out of whack. Not hard to do and if it is off it's not exceptionally hard to fix sinc you've already got the new belt on. If you pull the plugs so there's no compression to roll it over it's a lot easier to check as the engine won't want to rock past tdc.....Mike