The cell phone transmitters are invading the channels above 51; a preview of future problems:
Band 12 is being tested in Downtown Phoenix, AZ finally!
http://www.howardforums.com/showthre...ix-AZ-finally!
Band 12 is called the 700a MHz band (699 – 716, 729 – 746), and is most used by T-Mobile now
http://www.phonearena.com/news/Cheat...he-USA_id77933
https://en.wikipedia.org/wiki/LTE_frequency_bands
TV channel 51 is 692-698 MHz.
I wonder if a 4G-LTE filter would help you.
TVFOOL lists KPPX on real channel 51, but rabbitears.info and the FCC have it on real channel 31.
http://www.rabbitears.info/market.ph...&callsign=kppx
http://www.fccinfo.com/CMDProEngine....Number=2003423
Have you tried a double rescan?
https://www.google.com/?gws_rd=ssl#q=fcc+double+rescan