If you are getting misfire codes or even the dreaded random misfire code and have replaced coil packs and plugs to no avail here is what I can offer. IF the misfire shows up during a cold start up or a cold idle, you can hear it at the exhaust, sort of a puff puff, then you may have a bad injector. ...

There is also the fuel rail pressure sensor which if bad will cause the same problem and leave the random misfire code. The check engine light blinked at start up but once the fuel pressure built up it was fine. The mechanic that works on mine has just replaced two on later model R's in the last thr...