Did you install the X at the same time?... if you had to take the 02s out of the stock Pipe and installed them into the X you probably pissed them off. I had a similar problem when Sam@Coastal did my install. Had to buy new 02s... You should drive it for a day or 2. To give the Computer time to "re-learn" and if still having problems try 02s...
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Sn95' Tripple Black Cobra
Mods:Heads/Cam/Intake/Exhaust...just your typical stock mustang...