(Late Post on old issue, sorry, but only thing I have found that sounds identical)
I am experiencing the same thing in my 2002 WRX. It is stock, 40k mi.
SJwrx, did you get this resolved?
The problem is not consistent or really that reproducable, but it typically occurs in 2nd gear at about 1750 to 2250 RPMs with small even gas pedal. Stock option boost guage does not seem to jump up and down at all and is usually at about -.7 to 0 and rising.
Car will lope or surge/hesitate for just about 3 to 5 bursts. This is very slight power gain/loss and is not noticable to passengers on all occassions.
(There is also a "flat spot" in the accellerator, so that at high boost with full or almost full throttle, the car has power, but then letting off the gas to about <50% pedal suddenly the car seems to get TONS of power and really goes)
Are these issues related?
theories I have heard on surging: Gary's Hesitation Fix
Air Fuel mixture problem
Can you help troubleshoot on my own before I take it to the dealer?