do you have a check engine light on? The last Duratec tour I drive (SVT) that did that, it had a stickey IMRC. I know what you speak up... There is something in the duratec motors called "Secondarys" in a really basic sense, they open at about 4000 RPM's so you can have a more consistant power curse, if they didnt open after 4000 rpm's the engine would feel like a dog, very slow. I drove a CSVT with a failing IMRC, it would hickup right at 4000 rpm's every time you would get up there.
Some more information is needed though.. how many miles? Do you know when the last tune up was performed?