A misfire can be from many things.
The engine's knock sensors detect the misfire by noticing some noise or lack of a cylinder fire when it's expected.
Therefore .. it can be caused by anything that would keep a cylinder from firing properly.
The most comon:
Plug
Wire
Coil
Injector
Vacuum leak at injector o-ring
O2 wiring touching a plug wire (#4 or #6)
The less common:
Chipped piston
Cracked piston
Moisture in cylinder from LIM or other gasket
Vacuum leak from gasket (air related)
Typically misfires are worse under load, therefore it would be more readily found at that point.
I'd hook up a scanner and go for a ride with the 6 cylinders showing misfire info in front of me. Look for the one that lights up. That's where you concentrate your efforts.
