1996 pajero with high idle-sometimes

Hi guys,
thought that I would see if any of you lads can help me diagnose an idle problem before I throw money at it.
I have a 1996 NJ Pajero with a 3.5L V6. The engine runs fine, no misfire or low idle and runs smooth. the problem is that the engine seems to sometimes idle at around 2000rpm when I pull up at traffic lights, and will sometimes drop to around 1000rpm. It has been idling high off and on for a while with no engine light coming on, last Sunday the engine light did on for the first time and stayed on most of the way home but went off once the car decided to idle better. It does not do it all the time but is annoying because it is obviously harder on the brakes trying to pull her up. I read a thread on another site of a guy having the same trouble and the reply was that it is most likely the idle air control unit or stepper motor as some prefer to call it. I have checked all connections and carried out a resistance check on the stepper motor and all circuits read around 28-32 ohms as they should. I have pulled the stepper apart and given it a good clean but still have the high idle problem. I have another black electrical part that mounts on the above left of the intake (it seems to connect to the internal butterfly) and was thinking that this part may be faulty.
Can anyone help shed some light on my problem, I am not sure that a mechanic could help me diagnose the problem because the car runs fine most of the time and it would appear that everything is all ok if hooked up to a diagnostic machine. I can send pics of the parts in question should that help
Thanks for any help!

Cheers
Mark