I've got a 2002 WRX (secondhand) with about 140k miles. Recently (maybe the past two months) I've been having a strange problem.
Basically, when I'm gentle with the gas nothing is wrong. But whenever I put the hammer down (and by that I mean anything from "moderate acceleration" to "pedal to the metal"), I slowly accelerate followed by the engine racing and then falling back into the proper RPMs which is when I get full power. At first, I thought it must be transmission slip, but AFAIK, this would mean NO power to the wheels, and the car DOES accelerate the entire time. So then I thought maybe it was a boost problem, but I can hear it boosting at the appropriate time. So, somehow, some energy is being transferred to the wheels, the engine races (with no power change) the rpms go back down and I've suddenly got full power.