My sister has a 2000 V6 stang which has 75xxx miles and runs great. Car has been maintained religiously. A few days ago she filled it up with gas at a local Mobil station and it began running really rough. Shaking at idle and acceleration below 2k rpm. She put some gas treatment in and ran out the gas. Now with new gas a fuel system cleaner added and a new fuel filter it is still running very poor. Not sure where to go from here since it had no problem prior to adding this gas. Any thoughts?? Thanks in advance