V40 is playing up already... driving is fine, car goes like stink and embaressed a few cars...then ever so often the car is embarrassing me.

Sitting at idle the car drops revs as if it wants to stall, then picks up all of a sudden with the idle control valve. RT's showed no specific fault code.
I've wound the throttle cable in a bit to help, but it must just wind itself as after 10mins it is back to hunting.

The changes are 850 single mass flywheel from a dual mass one. We had wondered if the RPM sensor in the V40 need swapping for the one from the 850??
Any other ideas?