Running issue k jet 1.8 gti

I hope someone can help. So I moved to New Zealand nearly a year ago and shipped my cabby and all has been good. But the last few months it has not been quite right. When I start it up its all good and runs as it should but idles very very slightly lumpy. When I drive it along at say 2000 rpm in 3 or 4 th gear occasionally there is a slight judder like a misfire but it is not missing. I have checked the timing and its bang on and it has had new leads dizzy cap rotor etc I ever replaced the o ring on the idle needle as I thought it maybe sucking in air when it's hot. Also only run it on 97 octane and if I accelerate through the gears there is no issue and it goes like a rocket as it should it is only when I am cruzing around its plays up.
Any ideas as it is starting to do my head in.
Many thanks in advance.

martin. 

sounds similar to how my digifant was when co was too lean, aim for about 1-1.5% CO. have a good look for air leaks first tho

Hi rubjohny,
Thanks for your input I was starting to think it could be fuel related. I am going to warm it up tomorrow and turn the screw clockwise very slightly. I am going to go to the garage that done my warrant of fitness (MOT) and see if the can let me use there machine for 5 mins. Fingers crossed it sorts it.
Thanks.
Martin.

Hi all,
So after a little playing around my car is running well again. Rubjohny thanks as I suspected also the car was running a little lean. Fingers crossed alls good. Thanks again.  
Martin.