If you are at or over 100k I'd go ahead and replace the O2 Sensors, at least the Bank 1 sensor (not that this is normal maintenance), that is the one effecting fuel trims once the car gets up to operating temps, I'm guessing since it runs at a set pre-determined commanded rate by the PCM ((Open Loop)) it runs fine, but once it warms up the system will enter Closed Loop and the front O2 sensor will change fuel trims depending on readings determined by that O2 sensor.
If you are replacing the front then you may as well replace the other one as well.
Keep in mind I am no mechanic, but I have learned a little from a few smart people, and I'm still learning daily...
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA