1. Wrong, worn or incorrect gapped spark plugs. You should run only NGK stock range plugs gapped to .028.
2. Clogged injectors. Have the injectors cleaned or at least run several bottles of a good injector cleaner through the engine.
3. Carbon build up on motor. Real common on EJ205's. Get some BG Chemtool, GM top end cleaner or other type of carbon remover. GM top end cleaner seems to work really well.
4. Oil or dirt build up on MAF sensor. This causes an incorrect load reading which make for AFR corrections out of line. Misfires can result. Remove the sensor, place in plastic bag. Spray out with CRC MAF cleaner or NON chlorinated brake cleaner.
5. Vacuum leak. Check the turbo inlet pipe where it connects to the turbo. This pipe is famous for ripping and causing a huge preturbo leak.
6. Clean the ground cables that run to the intake manifold. These are sensor grounds and must be kept clean.
7. Cam timing
8. Excessive PCV blow by. PVC gases will effectively lower the octane content of gasoline/petrol.
9. Check valve clearances.
10. Lightweight flywheels or crank pullies. (phantom misfires)