I've noticed if I leave the 721 on overnight while paused in a PVR'd program, or even just left in the menu after the even is over the machine seems to flake out.
If I cancel out to go to live programming, no matter what channel in the guide I tune to the info for the program says I'm on channel 77, UKN. The screen other wise is black.
IF you attempt to turn the STB "off" via the remote control the unit will go off, but not turn on.
However, you can use the front pannel power button.
On the plus side, turning the STB "on and off" via the front panel "power" button seems to be a good workaround.
I suspect a lot of issue might stem from the STB not getting "powered" off overnight. Although it shouldn't matter since the STB is always on unless unplugged.