if the co is having problems at idle but ok at mid range i wold sugest you start with checking there are no air leeks in the inlet if you have a smoke machine its easy.
it does sound to me like you have a leek on the inlet.
i did one a couple of months ago that was just over mot pass on the co though pased the fast idle smoked it and found that the cam sensor on one bank had the rubber O ring missing replased and retested ok jon done.
hth martin