We have the same problem. All cable boxes (one is HD and one is just digital) go off at exactly 3 a.m. CST and the signal quits. After about 15 seconds the box comes back on, the TV comes back on, and the box hast lost its memory of all the programming. Everything on all stations shows up as "to be announced" even though we have it hooked to an uninterruptable power supply with a battery back-up (it did not lose power). The box acts as if the power was cut off. As far as we can tell, this happens EVERY night at 3 a.m.
I don't know what it is, but every few days all the cable boxes reset themselves at 3 in the morning. After this, the guide is blank for a long time. It also erases any saved programs and shows that I've set the box to change the channel to.