Could be that it's only firing on 3 cylinders. Suspect DI? Any CEL?
Sorry about that, but SuperSwede has explained it well. Fairly young car to be having these problems. Is it out of warranty?I am not sure what "DI" or "CEL" is... can you please clarify.
Do NOT let them use any other make and they MUST be resistor type plugs (hence BCPR). If they use ordinary plugs then you risk damaging the DI cassette. The DI cassettes were designed with NGK plugs as it senses the combustion through the plugs. If you use a different make then it won't work as intended.[/b]