It's been happening with some regularity on my 922. When it's down to 2 days, it also loses all OTA sub-channel guide info. The really odd thing is that one time 9-day guide was restored during a viewing session without any check switch or other action on my part.
When it happens on my 922... I can put it into standby sometimes and it will recover the EPG... but then it might be gone again the next day.
Usually happens to me around the end of the week for 1-2 days and then goes back to normal for a week. It doesn't happen every week either. I haven't missed any timers/recordings from it... but it is very weird.