Why is my Xbox restarting itself?

HDMI Has a Handshake when it is in use - That is why the console is restarting.

If you were to take the HDMI out of the XBOX while it was running it would reset to search for a new video source.

can you use PIP instead of changing sources?

Unplug every cable and put it back together again it could be a loose connection causing it to restart. After that ensure that the PSU (power brick) is in a well ventilated area if this gets to hot it shuts of to stop any damage being caused. If all else
fails contact support and set up a repair

I amazingly fixed it. Just as I thought demo I tried to download was corrupted. What I did was remove my hard drive recover my gamertag onto the internal memory. Then I looked on the internal memory data found the demo forza motorsport which immediately
started downloading by itself which I stopped and deleted. Then I put my hard drive back in and signed into live and had no restart problems. I then for good measure went to memory found that in my demo's there was one file that said "corrupted" and another
that said "Title something something" and deleted both. Now my xbox works fine. I dont know why the forza download I tried to get corrupted my hard drive, I should not have to worry about getting bad files from microsoft on their own network.

As above, could be video related. My xbox reboots when I change HDMI channel on the TV, if yours is random then a new HDMI cable would be a cheap thing to try.

Hello, I have a question. When ever I log in to a account it does what you have stated and I have also had this problem since I have tried to download a game. I don't know how to remove my hard drive and I do not want to lose all my progress and level on
my game. Does that happen when I take out the Hard Drive do I lose my level and progress in the game?