Shouldnt be anything with the TV... if anything like that.. it would be display/sound issues.. not the box rebooting itself.
I wouldnt think SIGNAL either.. that would cause say a LOSS of channels.. distortion, etc.. but generally shouldnt cause the box to reboot.
So that leads more to your last point.. location, etc.
- Its on a shelf by itself? Is there anything close around it/ontop of it? Is the spot its it, closes in with DOORS on the front?
(wondering/sounding kind of like an OVERHEATINT issue)
- How/where is it plugged in?
(could be a power related thing.. not getting enough voltage, etc.. that its flaking out. If its in a power bar.. try straight into the wall, just for a while to test if that is it.
I actually run mine through a UPS battery backup, to help prevent any sort of power fluxuations)