engine hesitation at operating temp

[CTDiesel at aol.com]

4kq, between 1000-2000 rpm when getting up to speed the car hesitates
significantly until it reaches 2000rpm and then acts like it opens up with
lots of power.  I finally fixed the cold start problem with a new water temp
sensor and but the air/fuel mixture back into closed loop.   The O2 sensor
does show the correct flip flop of .2 - .7 volts.  What are the chances its a
"lazy O2 sensor?" or is it something else?

TIA