We never try to get these engines to idle down to 2000. There are just too many factors involved.
We keep them at 23 or 2400 where you are just into the regulator and things are a lot more stable. Some engines will do the 2000 idle ok, but most will not.
As far as the problem engine is concerned, it sounds like it needs to have the piston mod done. Maybe you ought to send it in so we can have a look at it.

Dave