Dan, this is a known issue with 13 and the 151 (had one for over a year).
Every now and then, something goes off at the station, and they need to reboot something or another. Drop Frank Liley a note, and he should be able to take care of it straight away. I had him reboot several times when I owned the 151.
For what its worth, what you don't want to do when you notice they're gone is re-scan the channels. Then you will be hosed for sure.
When I had the 151, I could tell things were headed south when it would take a minute or two to lock on to 13. If I saw that, and then re-scanned, I would be dead until the reboot.
If you should notice that you are having trouble locking in, thats the time to e-mail Frank. After he reboots, all will be well without any additional action on your part.