I agree with Z’s hypothesis, that you’re losing signal and it is reverting to the .1 subchannel. I’ve never experienced this on any DTV set I ever owned though — not as you are describing it. The closest I’ve ever seen happen was watching CBS-2 on 48.3 (WMEU-CA Chicago) I left the room, came back and the TV was now on 41.1 “ABC 41” out of Battle Creek, MI (WOTV). Perhaps when a plane goes over, it reflects bits of some distant station using RF Channel 23 like WCIU Chicago — 2.1 WBAK Green Bay in on RF 23 for example — the TV gets momentarily confused and then dumps back to 26.1 when the plane moves on... Total guess. The only way to fight something like that is with a more directional antenna.