ski

I'm betting thatthe cause of your problem isspark plug fouling from the octane booster. There are numerous posts on everyMustang forumstating that other S197 GT owners had performance/misfiring problems after using octane booster, and replacing the plugs solved it evenwhenthey did notlook fouled.
If you still haveyour old plugs and they performed ok and were not exposed to any octane booster, then run any remaining gas containing octane booster out of the tank, and reinstall them. If your car performs normally, then install new plugs.

canton

So it's fixed. Not sure exactly which of these it was, but spark plugs are the most obvious culprit.

Ended up ordering an X3 from Brenspeed and getting it tuned properly. Had the oil changed and put a motorcraft filter on it, also did the fuel filter. Finally replaced the sparkplugs again last night and it's running like a champ again.

Thanks for the help!