[thstone]

Misfiring is usually related to an issue with the engine timing or fuel/air mix. Usually, you'll get a CEL if the fuel/air mix is off (this is what the O2 sensors monitor), so the first place to look for a misfire with no CEL is the engine timing:

The input comes from the crankshaft position sensor. If this sensor isn't working reliably, the timing will be off.

The output is through the coil packs/wires. If either of these aren't working properly, the timing will be off.