Sometimes it is a defective unit, and sometimes it is overheating.
My 622 used to reboot, until I added a small exhaust fan to the left side. Since then, I have not had one reboot (aside, of course, from the intentional 3am daily one).
Had to call technical support for the very same reason. Ours started to reboot once in awhile...until the unit went loco and kept rebooting in a constant loop. I ended up swapping it in for a new (read: refurb) unit.
Mine is sitting on top of a table with lots of air flow so overheating should not be a problem. I have another 622 in the bedroom on board (over the carpet) underneath the dresser. So it is completely hidden. It works fine.
A number of people reported their 622s rebooting on their own very recently and mine was doing it too. A replacement is on the way ... but mine has stopped doing it now ! Anyone else seeing this too ? Really makes me wonder if there wasn't something on Dish's end causing this !
I had one keep rebooting, tech sent out another one. I unplugged the bad one for 24 hours, plugged it back in and it worked fine! When ( replacement) came, sent it back . Bad move, 2 days later rebooting started back up and would not stop! Had to have another sent out!
Mine started rebooting for no reason, you could be watching TV and boom it rebooted. My receiver was in an open space so over heating was not the reason but it's a valid one. It's also on a UPS. I had it replaced and so far so good except that one night the new one locked up while watching a recorded show. I just hope it was a one time thing.