If you have dirt in your fuel, I'd try the insurance route first. A) Vandalism or B) Bad fuel... let the insurance find out which one. Also, let the dealer show you the said "dirt".
It's hard to tell sometimes. Sometimes you get lucky and the tuner will put a sticker on the ECM. Also, perhaps someone will chine in here with tuning that can hopefully shed some light on those characteristics can lead you down the right path..