Thermoquad question

Must be just in the readings, I'm thinking... What are the symptoms if an O2 sensor is bad? Does it just quit? Mine isn't that old miles-wise. AEM A/F meter seems to be working fine, other than when it reads way lean part of the time.

Engine build is on here. (freshen up) ...some basic info in first post here.
Yes right now running stock spring under rods. Tried none, and a lighter hardware store spring but (best I can remember!) seemed to go too lean on part throttle around town. Put stock back in and was doing fine till lately.
vacuum is 10-11" idle, around 15-17" at cruise.
Solid lifter Lunati with EDMs, 235/243 at .050 and lift of .526/.546.

Big TQ. Most all jetting and settings according to the old Mopar bible and/or old posts here from "demonsizzler." Everything dialed in carefully. NGK BP5ES plugs really are spot on, and like I said it's running great. But something is haywire!!

Either fuel supply (which I doubt, or it would show up in performance,) or in the Bosch O2 sensor and/or gauge???? Not sure what else it could be. Ideas???