I too have a similar kind of problem with my ST200, it just kind of hesitates at around 4k & 5k, almost the same sort of sensation as if you drive through a big puddle, just hesitates a bit and is then off again.

As far as I can tell my IMRC is working, it opens the second butterfly at about 4k.

What I seem to find though is, it seems to drive better when it's cold, ie when I jump in it for the first time in the day, it seems to accelerate better and pull a bit harder and not really hesitate so bad. Kind of leads me to thinking it MAY have something to do with the IMRC still, as the heat from the engine may be having some adverse affect on the IMRC unit.

If you fix yours mate, let us know what the problem was!