Where is the computer and where did you get this done?update
Firstly thanks to everyone who's given feedback on this issue
Thought it time to update this thread now.
I've been to Mr Nissan and they found the nissan bulletin giving the same info that GQ banger gave (thanks for that).
I got to sit in the car while the mechanic did the job. It was really quite easy. He unscrewed the computer from its mounting points and while consult II was monitoring the idle revs (it was idleing at 750rpm) turned the screw under the sticker that does'nt have nissan printed on it. As he turned the screw clockwise the idle reduce to about 700rpm and then increased up over 800rpm.
We set at 800rpm and remounted the computer. The car now idles so smooth!!!! It also has less lag at the lights.
Having said all this I'm not sure if it has fixed the actual problem or just masked it, but will monitor and get back to you all with the true result.
PS. Does anyone know how to access these Nissan bulletins which can hold so many secrets?? Talking to the mechanic when he did the job and he said most of the dealerships wouldn't know about these types of fixes unless they had actually gone looking for them because Nissan doesn't advertise that they are available. He also mentioned that my car (an '01 model) is now old and the dealership are so busy trying to keep up with the new stuff that they just don't care for the older vehicles