So my 1999 RS threw a CEL the other day, and then I noticed that I am definitely not running right. The CEL is P0420 which is "Catalyst Efficiency Below Threshold (Bank 1)."

Never had this problem that I know of and was wondering if anyone else has. I am pretty much stock with an intake and headers. I noticed that when I start the car, the RPMs jump to between 1500-2000 and then after a few minutes finally head back down to like the 500 RPM idle range. I am also having heavy hesitation in every gear between 3k and 4k RPMs but nowhere else ouside of that range. I will be fine getting up to 3k... will hit hesitation hard (the car stutters and I can tell there is obviously a problem)... but then when I hit 4k, the car gives a little kick and I am at full power. I just wanted to get some opinions before I take her to get looked at seeing as how I don't have the time to wait around at a dealership for hours on end. It would also be nice to be able to get any parts before I have it replaced so I don't have to spend a grand to fix a $200 problem.

I changed the plugs and wires about a year ago so I don't think that is the problem, but somebody said that the coil pack may be causing a misfire

Could it be the catalytic converter?

I have also been told that at times headers will throw this CEL b/c it throws off the O2 sensor, but I have had the headers for about a year now and this is the first time it has thrown the CEL and the first time that I've had the hesitation.
Thanks for any help guys