Injector cleaner for PETROL is ideal. The LPG cleaner I think is the same stuff, but twice the price. Injector cleaner for PETROL works for me.
If cleaning has made that much difference then you have 50/50 chance of being lucky. The dirt in there was loading the stepper, and messing-up your smooth running. You have found the cause, now all we need ask is, 'Has it damaged the stepper?"
I am convinced that you do not need to remove the manifold, I had forgotten, but I have heard from two reliable people now that you don't need to do this. Be careful not to wreck the rubber mounts with too much force.
There are 3 or 4 things that kill V70 P1s. PCV is a whole other subject, it is a V70 P1 killer. I think P2 as well. Along with a Gibbons Flush to your P1 or P2 Autobox, if not done this will send your car to an Italian scrap-pile.
https://www.youtube.com/watch?v=eEZzqyu2a6c
My post on the subject. I can assure you, my knowledge of Necam and PCV is as good as you'll find. You don't need to take the inlet manifold off for Necam , but you should soon to look at your PCV.
http://www.volvoforums.org.uk/showthread.php?t=252505