Spark plug wires is a good bet. 5 and 6 can cause special problems on 5.0s since they are next to each other in firing order. With at hot coil and bad wires, you can have a crossfire every now and then. You can't just look at wires, either. It's possible to accidently loosen a crimp during a tune up. Wires tend to move around on acceleration and decelleration. One could be shorting against the block only during those instances.
When's the last time you did an actual tune up? Cap, rotor, and plugs? What about a fuel filter?
You should only check plugs directly after doing a run.
Replacing the ACT, ECT and O2 sensors is probably a good idea at this point. After you replace all of the sensors, reset your computer.
You did say you had this problem before the ADS chip, right?
Oh, as for his stereo. I'm sure that he's tried turning it off when test driving. I'm pretty sure that's not it. I was actually gonna tease him about the subwoofer messing with his knock sensor, but Mustangs don't have knock sensors. So how the hell do timing gears set it off?
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351W 89 Mustang GT Convertible
[This message has been edited by jimberg (edited 06-22-2001).]