Pontiac Repair: 1999 pontiac grand pre 4.8 missing at high rpm, shade tree mechanic, muffler shops

Question
I have a 1999 pontiac grand prix 4.8l engine non supercharged. The engine has a regular miss only above 4000 rpm. I'm only a shade tree mechanic trying to fix my daughters car. I've taken it to several mechanics and all their responses vary. The codes were P0300 P0131 and P0420. I have taken the following steps: Replaced fuel filter. Removed fuel line to rail to check fuel pump output. changed plugs and wires. removed wires from coils to check to see if coils were firing. dropped exhaust at headers to ensure cat converter wasn't clogged. replaced tps sensor. removed air filter. checked for vacume leaks. disconnected battery to reset codes/get engine light off. I know I probably need to just take it to the shop but they scared me when I took it there. The mechanic started saying it could be a number of things and that it could take a long time to figure out. In other words it could cost me some big bucks.  I would really appreciate any advice.  Could a bad O2 sensor cause this and is there any way I can check the O2 sensors?  Thanks for any help you may be able to offer.

Don

Answer
Hi
Is the miss happening while revving in park or nuetral?
IF SO your hitting the rev-limiter and that is a NORMAL condition.
If it is doing it driving down road, what speed is it going?
If it is above about 109mph, that is also limiting program in the pcm.

The p0420 is the code for the catalytic converter. You can take it to MOST muffler shops and have it checked for free or almost.

If not, observing the data on a scan tool whill show which cylinder is missing. From there you can pinpoint problem to fuel, fire or mechanical.
I Hope this helps. Follow back, if needed.
Good Luck
Pawl