Question about drifting AFR numbers

[FD3S]

I have an NGK AFX with the NTK sensor. I'm not sure how old the sensor is, but I have free-air calibrated it several times. It was never significantly off, I just wanted to make sure of the calibration. Originally, it was pretty far back in my exhaust system, after the y-pipe, but I've moved it near the driver's longtube collector with no change in behavior. The positive wire is connected to a switched 12v source, and the negative was originally on the battery but due to noise I moved it to the chassis which smoothed out the readings quite a bit.

Sometimes, gently dropping back to idle, my AFR will hold steady around where I have it set to. A lot of the time, however, my idle AFR will drift lean until it pegs lean and holds there. Once I tap the throttle/start moving it'll richen up. The other day, from starting the engine cold to driving to operating temp, my cruise AFR was maybe a half-point rich. I pulled over, turned the engine off for 10 seconds, restarted it and it was back to where it should be while cruising.

Are these symptoms of a dying sensor? I had tuning issues last summer where I'd tune for wideband stoich in OL, put the car back in CL, and my fuel trims were +/- a few percent but my wideband was constantly a half-point rich from where it was during OL. I'd like to get this totally sorted soon, and I'm debating replacing the sensor or borrowing a second wideband for comparison purposes.

I don't have logs on this laptop, but I suppose the questions might be general enough for more generic responses?

[RedWS600]

My last two wide band sensors (over about 12 years) both ended up sticking to around 17:1 AFR. could see the other O2's switching well, no matter what I done with the throttle, I couldn't get the reading lower. Changed the sensor and all was good again,

[FD3S]

Driving home today, I'd cruise at a slightly swinging ~14.7x (currently OLMAF), come to a stop light, idle would drift/peg lean, then when I started cruising after the light changed it'd hover around 15.3x for a while before eventually come back down after some time/shifts/etc. I can't explain why the AFR would just shift around like that at what seems like random.