There is a thing called the Intake Manifold Runner Control (IMRC) which opens the secondaries on your 2.5L at around 4k under moderate to heavy throttle. Depending on what you are doing at that point depends on when and how much it opens them.
These things go out after a while which is a pain because your car is all of a sudden slow and inconsistent with it's power. I am in the same boat right now. Mine comes and goes as it pleases and ultimately I am going to have to replace it as you probibly will.
It is located under the 2.5L Duratech black plastic cover on the front of your engine. It is the piece that goes around your oil fill cap and has 3 screws holding down. Take this off and you will see a metal box with 4 philips screws, a plug, and a cable. This is your IMRC. If you would like, take the screws out of it and crack the seal on the lid to take a look to see if maybe your gears are not meshing or the linkage is screwed up. I fiddled with mine and it seems to be working better than before but it doesn't feel like it's opening all of the way now for some reason.
Anyway, check that out and see what you can find out.
