Narrowband not agreeing with wideband

I've got a WBo2 gauge in the dash and I always glance to confirm I'm 14.7ish. I'm having intermittent issues with it dropping to 13 or even 12:1 during normal light throttle driving and idling. The NBo2 is showing 100-200mv and BLM/INT are adding fuel. (Tune has been almost spot on.) I put another o2 sensor in and even swapped positions for the NB and WB and still get the same results. This leads me to believe it may be in the wiring from the sensor to the ECM. Has anyone run into something similar before? The wiring harness is 28 years old now so it seems like a distinct possibility.