Well my 03 bonneville ssei just started missing 2day. I have a k&n intake, flowmasters and a 3.4 pulley. replaced the plugs with ngk tr551X and it is not getting any better? Before it was at low Rpm'* but when i changed plugs it ran fine down low but around 3000-4000 it misses quit rapidly. 128 misfires in 10 seconds is no good. i found this out but a gm specific tech 2 scanner. oh yeah I only have 47000 miles? This sux!

What do your wires look like?

At that mileage, plug wires as mentioned are something to look at. Check to ensure the back wires aren't touching the O2 sensor wiring. (this is a common one).

After that, I'd suggest testing the resistance of the coils with the writeup in Techinfo.

while checking the resistance on the ignition coils, check for corrosion on the terminals that may be hindering spark performance. If there is much noticable oxidation, clean them off wityh some steel wool or medium grit sandpaper and apply dielectric grease on them before reinstalling the wires

Clogged cat?

Well what i found last night was no good. The k&n intake had rubbed on the pcm harness and wore through the #4 injector wire from the pcm. I fixed that prolem but now the misfire is at high rpm not low. The only other thing i changed was the plugs to ngk iridiums # TR55IX. I just pulled the 3 back ones out and they are not burning correctly so im going back to 41-101 ac delco and belden wires right now. I will post what happens shortly.......... Cats fine too.