other possable causes if not the plug may be coil pack breaking down or the plug lead you say it runs fine for 30 secs so it wouldnt be a valve they eigher seal or dont compresion test would be my next check could be head gasket gone near the plug when the engine warms slightly it starts spraying a fine watery mist over the plug causing it to misfire im a mechanic so it dont cost me to comp test but to try the coil pack or lead it does not unless you got a m8 or a rs owner that could swop yours out breifly unlikely to be the feul injection side its getting feul to the others be different if it wasnt getting feul at all when you say it runs fine for 30 secs no disrespect to a trained ear might be a different case it should be one of those