hesitation

[Brandon Rogers]

Guys - 
Asking a for a friend becaue it's driving _me_ nuts.  The 3B is chipped,
otherwise stock.  On acceleration the motor hesitates.  Almost feels like
small surges as the hesitation comes and goes - but it never feels like the
motor is really pulling as hard as it should.
 
Aftermarket boost gauge shows 20psi.  Plugs are low mileage F5DPOR.  He's
tried two different ECUs - same symptoms.  Nothing consistent in terms of
codes - has thrown "low voltage" and "O2 sensor range not reached" - or
something to that effect.  
 
Ideas?
 
TIA-
 
Brandon 
'84 ur-quattro 20Vt