Been watching this thread with interest.  In April had very similar
problems.  Turned out to be two part.  First, a bad injector, running rich,
caused the ECU to lean down all injectors.  Second, maladjusted throttle
switch.
Problem was defined by disconnecting the O2 sensor, ran great then but very
rich.  Replaced O2 sensor, problem still there, isolated to the #2 injector,
replaced injector, most of problem solved.  Adjusted throttle switch, ran
like a dream since.  Some indication now of throttle switch slightly out of
adjustment but not a major issue.

I only mentioned this because no one has mentioned throttle switch
adjustment as possible cause of the 'long hesitation when stepping on the
gas'.