5KCSQT Hesitation

["Bill Rowe, CFO" <browe@gtminet.net>]

Experiencing hesitation only at full throttle, whether cold or fully
warmed-up, going from 1st to 2nd gear and getting poor gas mileage....
approx 18-20 mpg.  Otherwise car starts, idles, runs great.  Just replaced
fuel pump, fuel filter, plugs, cap & rotor, K&N air filter.  Bought car w/
101k miles 5 yrs ago from orig owner and now has 150k miles.  Car has been
well maintained by myself and previous owner.  Plug wires have never been
replaced since I've owned it but doubt that's the problem cause they still
look good and have seen no arcing in the dark.  Don't suspect warm up
regulator.  Boost is fine... 1.3 in 2nd to 3rd gear and 1.4 in 5th.  I
suspect the oxygen sensor.  Is there a fool-proof and relatively easy test?
What else would account for the symptoms I'm experiencing?