My misfire problem started small. I really thought it was just a hiccup of the transmission or my foot slipping on the pedal a bit for a long while. I'd just get a little bump once in a while. Say, every 500 miles or so. Then, a couple times it began bucking really bad with NO power. Realised later that the CAT was getting red hot.
The problem is very very erratic. I have driven 20 miles to work thinking it was ok only to have it go bad half-way home. I have had it go bad on cool mornings and hot afternoons. The only somewhat constant is that I can pretty much be sure that it will go bad if I really stomp on it for a while.
Changed plugs and boots.
Inspected hoses and listened intently for vacuum leaks.
Found intake past the throttle coated in thick oil. Cleaned it out with throttle cleaner in a can.
Decided the intake had a vacuum leak, so I changed intake gaskets. During that, I found that the entire intake manifold was coated in the oily mess. This oil I am 99% sure has come from the PCV vent in between #2 & 3 intake ports. I think this b/c the oil mess was evenly dispursed to all four runners and did not reach (was not from) valve guides.
Took it to the dealer and paid for a diagnosis. They said it was the ICM. Replaced it today and found that they were wrong. Went back to talk to the dealer shop about how they concluded it was the ICM. Apparently the mech just followed Service Instruction PIP3433D which says that (unless there is a short) P0300 can be fixed by replacing the ICM.
Looked further through the forums here and found many people who had either the ICM or coils causing their problem. Also found a sticky posted by Maven with the ignition schematic. Using this, I found the correct pins on the coil assy to probe with my Ohmeter. The seconday coils were both reading in spec (4-8K Ohms) at 4.4K Ohms. Cylinder 1-4 Primary coil was in spec (.4-.8 Ohms) at .6 Ohms. Cylinder 2-3 was initially in spec at .6 but when I touched the probe to the pin again it read .8 Ohms. I touched several more times and the resistance rose quickly to 4, 20, 50 and finally in the hundreds of Ohms.
Replaced the coil assy and the car no longer misfires. Unfortunately, it is throwing code 420. I assume that I destroyed the CAT from all of the misfires.
If you get misfires detected, checking resistance on the coil assy is a VERY easy proceedure. Takes only about 20 minutes.