Short answer; probably a signal problem. Check connections and wiring from wall > box and any other cable wiring you know of in your house. If that doesn't work, you will likely need a Cox technician.
"Temporarily Off Air" is kind of misleading in that it is rarely temporarily. It just happens to be the default message that Cisco made the box give when it can't tune into a channel. Picture the box being like a radio, but instead of the channels dial going in order, its ordered in blocks of frequencies. Sometimes you can just have a signal problem on certain blocks or frequencies. Also certain blocks can be weaker then other blocks, just like certain radio stations come in clearer and if your signal is generally low because of a signal problem, sometimes it only effects the normally weaker blocks. Resetting the box makes it scan the different channels and sometimes lets them come in better, but if the overall problem isn't fixed, the symptoms soon return. Other things can cause the "Temporarily Off Air" message, but signal problems are the most typical.