Have you checked the movement of the plunger in the metering unit? If its a bit gummed up it may be slow to respond and only when the revs are up and demand is raised is the flap beginning to lift enough for the engine to rev cleanly.
I know you say you've tested everything but just make sure the cold start injector isn't jammed open.


Regardless of root cause you're still going to need to get the fuel mixture right for this to idle properly. A few too many degrees clockwise or counter and it'll be wanting to die regardless if everything else is perfect.