I still like the adjacent channel explanation. You have a low-power K18JL on real channel 18, which is right next to KPHO on real channel 17. It is possible your Wally got "confused" on the OTA scan, and is sometimes displaying the strength for the wrong channel while you are watching KPHO. I used to have a similar issue (displaying the strength for the wrong station, with a signal loss message on the screen) on one of my Wally after a bad OTA scan messed things up. Another OTA scan (at a time when all of the stations were actually coming in with a stronger signal than I normally get) fixed that problem. So, a re-scan may not hurt in your case, either.