Yeah, I didn't want to post in this thread last night.
But I would disconnect the SAT inputs and see if the unit keeps rebooting. (If not, also run a check switch at this point) because his setup may differ from yours and the 921 is corn-fuzed.
If there was anything DVR'd, try to play it while the unit is disconnected from the SAT inputs.
Next I would try unplugging it for at least 1-2 minutes if that didn't work. Give everything a chance to settle back in.
My 921 has never had heat issues and it sits in a fairly unventilated cabinet. It is unlikely that heat is the cause, and if it were, it should only do it after several minutes or hours of 'ON' time.
Vibration should ONLY be an issue if moving/banging the unit around while it is plugged in/turned on. Even powered off, the Hard Drive still spins so moving the tuner 'can' possibly damage it.
Polarity: This is a reach at best and should have no effect, but what if the house wiring is reversed or maybe there is an open ground loop somewhere between the dish antenna and the 921. This is also unlikely, but I added it for completeness.
Lastly, if this
wasn't a close friend, or if it was an ebay transaction, there is the remote possibility you're getting scammed. This happens when people buy identical equipment to what they have that is not working and then try to get the seller to believe theirs arrived DOA. Make sure the RCVR and Serial #'s match what you had. THis is a long reach, but it ddoes happen.
Thats all I can think of.