What actual AFR readings are you getting at this so-called 'lean spot' and at how much is the 'part-throttle' condition? 14-15 is fine at cruise and light throttle.
People get all hung up at keeping an AFR around 13 all the time and it is just a waste of fuel; at part throttle, the cylinder filling is only partial and the cylinder pressures are much lower so pinging and detonation is very hard to have occur unless the timing is waaaaaay wrongly advanced. FYI, the Chrysler Lean Burn system would run up into the 18 range for AFR.
The O2 sensor is a great tool; just don't let things like this fool you.