It could be the O2 sensor too. Mine would stumble when driving and it would go away at wide open throttle, until the sensor finally got bad enough to set the fault code. I would see if you can get someone to actaully look at the engine data while it does it, and not just look at the fault codes.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs