This kind of crap happens to me all the time. It's horrible.
It's happened at multiple addresses, multiple digital boxes etc... The most common problem used to be that as an On Demand program was starting to load, the box would just spontaneously reboot (assuming I didn't get a "ROD is temporarily unavailable" message).
Now with the 8642, the program just fails to load and all of my channels say something like "This channel is unavailable at this time" until I reboot.