Stang Bang98

anyone familiar with error P1131? i have this code, and FORDs garage told me they couldnt find the problem...after charging me 90 bucks.

Stang Bang98

So let me ask this...if an O2 sensor is bad, and throws code saying its running lean...the O2 sensor has the ability to try to "compansate" and dump more fuel to correct a problem that isnt there? i was throwing a code saying it was running lean/bank two. air fuel has been tested and is right on the money at idle. it hesitates and buckles between gears and expecially at around 45-55 at constant speed. (as though its not sure when to switch gears) the idle is sometimes down to 600rpms and sounds like its dying. other days the idle is fine. I believe there is also a aftermarket set of pulleys on there. could a problem starting also be related to that alt. pulley not getting charged? ive heard bad things about them. Its a 98 gt auto, 4.6 sohc. Please help asap, im debating buying and trying O2 sensor tomorow. (i know air fuel is good, and theres no major air leak. EGR has been replaced, MAF has been checked. aftermarkets include bbk intake with k&n filter, american thunder flowmaster axle back, 9mm race wires, and motorcraft plugs. possible aftermarket pulleys) I appreciate any help, thank you.