[MetaJacko]

Is it possible there is too much pre-load?
I ask this becouse you say it runs better cold. As it warms up I would assume the expansion sort of adds more pre-load. (A bit like the lash being used up as an engine warms with solid lifters)

Also, a friend told me once that you can have problems at higher revs if the exhaust is restricting the flow. He described it as the power dieing quickly as you have described.