Rough Idle, running - Stepper motor? O2 Sensor?

Hello All,

Over the past year or two, the "Check Engine" (EFI) light in my '93 Range Rover 4.2 has occasionally come on while idling. Sometimes this comes with a rough idle, sometimes not. In the past, I have simply reset it by pulling the negative lead from the battery and all is well for at least several weeks. The first time it did this, I took it to my mechanic who put it on the test book and it came back with an "idle too slow" reading. At the time, he said it must be the stepper motor and that I should change it if it gets bad.

OK, when this happened most recently, the rough idle didn't go away and in fact yielded some occasionally coughing when the RR was under strain. However, when the check engine light came on the whole system immediately reverted to normal smooth running and stayed that way until I reset the system.

I assumed that the stepper motor had finally completely gone and replaced it this evening. Things seemed a little better but still, the idle was a bit rough and it seemed a but underpowered when under strain going up a steep hill. Then, after a half an hour, the check engine light came back on and, like clockwork the engine was running perfectly and able to take the steepest of hills with aplume.

Sooooooooo....Now I'm guessing that BOTH the stepper motor and a sensor like the O2 sensor were bad. Replacing the stepper motor DID improve things. However, when the check engine light it coming on, it seems that the system is ignoring some bad input somewhere and that makes me suspect a sensor.

Am I on the right track? Does this make any sense at all? Any ideas will be appreciated. This Rover is a stalwart beast that has given me very little trouble in the past and my heart tells me that it is something minor - please, let it be so!

Thanks,

Arden