you may have to get your dish repointed or get a larger dish for your locals if you live on the outer edge of the spot beam, oh and some of the locals have been recently converted to spot beam that may be why you are getting the problem now, or the locals have been switched to the new 118.7 satellite
I'd ask the same question. my 501 has been having problems ever since WFAA moved to spot beam last wednesday (May 17th). I have a 4p-5p Oprah daily timer that continually gets 'channel not found' error during the recording and ends up recording about half the show. Power button reboot will sometimes fix it, sometimes not. Also, see the popup coming up on KXAS (channel 5) which also moved to the spot beam recently.
My 942 running off of the same DPP Twin works just fine and I ended up setting up a backup timer on that box as well... and it recorded without incident.
I was beginning to wonder if my 501 was trying to kick the bucket or not.