I had a similar... but much more severe... rise and hang problem with an '02 SVT Focus. Started around 10K miles shortly after a computer reflash done by Ford. It would rise to somewhere between 3K and 4.5K RPMs and totally stick or fall off slowly after several seconds, but only when warmed up. Really hard to shift like that in traffic, and I told the dealership there was no way the clutch wouldn't die early from such a lack of throttle control... that they'd better be willing to replace that under warranty if the problem continued. Of course, they balked at the idea (wear item, you know).
By the time I got rid of it (so many other problems too) we were starting to focus our attention more and more on the Idle Air Control and/or the new computer program or maybe a combination of the two. Several times at the dealer chasing it down (the last time almost a week) and they never managed to get it totally under control, but it got a little better after an IAC replacement.
Maybe start there.
I sure hope you have more luck with this sort of thing than I did. What a gremlin that was. I ended up dumping that Focus back on Ford to get a new Mustang. A long, painful story, but the 'Stang has not had any real problems so far.
