Need help solving following problem:

Van in2nd gear at around 3200 rpm, experiences uneven power, a kind of
surging;  not pronounced but noticeable.  Other than at this point, the van
runs & idles perfectly both when cold and warm.Disconnecting the 02 sensor
eliminated the problem altogether.  I subsequently replaced the 02 sensor
with a new oem one but this did not solve the problem.  The AFM is new &
meets spec.  The throttle switch has been adjusted to spec.  The coolant
sensor meets spec. (tested when cold -- coolant at 50 farenheit).  What's
left?  Vacuum leak?  Fuel pressure, pump or clogged filter?

Any ideas much appreciated,

yann buchanan
ynb@att.net
87'wolfweekender