Just thought I would share this info with you guys. I had a customer w/an 02 WRX w/ misfire codes for each individual cylinder(po301-304) as well as the random misfire(po300). The car ran great and never actually misfired on my lengthy test drive. The car had 108k miles and was bone stock. The previous place working on it had already replaced the plugs twice (in about a 4 month period) and suggested it needed further diagnostics and compression test which would have been several hundred more dollars. One thing in misfire flow charts for any given manufacturer is to check ccam timing so I got approval to change the timing belt in the process b/c it was original anyway. The driver's side cams were slightly out of time and the belt replacement has fixed the car. The same thing went on w/ my car as well w/ a lower mileage belt but my CEL is always on and I thought it might be due to the 13lb. flywheel, but it's been about 2500 miles and no misfire codes. So do your maintenance and save on diagnostic fees!
whta kinda milage were you talking? i bet i'm in the top 5 here of high milage 04+ models...something i may need to worry about...