high boost stumble

["Beer, Jerry" <JBeer@BooseCasey.com>]

Ok, list traffic is light, so maybe someone can help. Patient is a 1991 200q
20v, 160k.  IA3(about 2-3 years old), 2Bennett k24/26 hybrid.  When set to
16 psi per VDO via some spring shims(car gauge reads 1.8), car pulls
cleanly. If I shim the spring to pull 20psi (car gauge 1.9-2.0), car has a
flat spot anywhere between 17 and 20 psi.  Hard to describe- feels like a
very soft rev limiter. For you "old timers". feels like a stuck needle in a
needle and float motorcycle carb setup. If I get go pedal off the floor a
little, it clears right up and car surges forward. The boost gauge seems to
hold whatever boost level the car is at when this occurs.  Seems a little
worse when car gets hot from repeated runs at boost, but it won't occur
every time. Most runs are in the higher gears.  Both knock sensors recently
replaced, plugs (F5dpor) relatively new.  WOT switch?  Fuel pump not flowing
enough? (not terribly noisy).  Other than this problem, the car flies. No
fault codes read out. Any WAGS?  TIA
Regards,
Jerry