This is just a case of temperature inversions occurring at inopportune times. When you have this problem with KOVR you should check KTFK. It's on RF 26 right next to KOVR and the antenna is on the same tower. I've found both stations to go up and down in unison. If KOVR is bad I would expect KTFK to be bad too. KXTV is also on the same tower but VHF can act different from UHF even from the same location.Antenna is a deep fringe Weingard with a 7777 preamp on a rotator mounted about 30 agl. Last night when KOVR went bye bye, I changed over to chl 10 and the signal was almost as high as my Sony TV signal meter can go. Click back to KOVR and it dropped by over half.
I have the same lack of a back-up CBS station. KION is too unreliable and is subject to interference from KEMO on 32 and KPIX is too weak here to be reliable.
I'm still curious if you have LOS to Walnut Grove because LOS is usually quite reliable reception. The fact that your signals go up and down so much suggests you have at least 1 edge.