We are running 95RON ( European Standard rating ). The service advisor did show me the little container of addictive, but I wasn't paying attention and so don't remember what it was exactly.Very odd. It is possible and even most likely you have bad fuel. If you dont mind me what octane are you running? What type of additive did they put in and how much?
If this continues, I'm taking her back becuase none of my previous cars idles like this.