Do you mean it feels like the power at a constant acceleration rollercoasters?
What is your Boost gauge doing during this period? Is it steady or fluctuating?
Does it do this only under boost or can you replicate it steady state reving the engine?
Have you performed a boost leak test to at least the pressure your turbo normally produces at the hot side of the intercooler, say 1-3 psi higher than read at the manifold?
I do not think in this case any TB or Biss leak is large enough to cause this. Usually I have found this to be a problem where a clamp has a partial seal to a certain pressure which bursts and then reseals once pressure drops below burst level. Knock has typically presented itself with a single drop in power rather than a fluctuation in my experience. I have never seen quick knock correction cause power fluctuation except in minor knock circumstances and most of those were noticeable only on a log, not the seat of the pants. Another possible condition which I could see cause a fluctuation, if that is in fact what you are experiencing, is a leaking or improperly sized injector, which would add unwanted fuel increasing RPM's. this might be almost unnoticeable during idle due to the lack of fuel used in idle situations. A boost leak at the injector would have to be fairly large to cause noticeable fluctuations. The clutch is possible but you would probably smell it to some degree.
/brox