Looks like I may have found the answer to both of my problems (corrupt recordings and missed recordings). It seems that one if the satellite inputs was not connected properly. I got the clue when trying to change channel and getting a "no signal" error (I was recording the channel). I have to say that I find the push-and-twist connectors on satellite cables a bit frustrating - they just don't feel robust enough, and are prone to unravelling if the cable is accidentally pulled on stepped on.