Comcast picture format keeps changing

On my Comcast box, I went into the setup (by powering the box off and pushing menu), and selected 1080i as the output. However, later on I notice it will switch it to 720p on its own sometimes and stay that way. I don't power off the box or anything, it just changes it back to that on its own. Any idea why?

What type of box is it? and how is the box connected to the TV?

Its a Motorola DCH-3416 box. Its is connected via a HDMI cable to a Yamaha 663 receiver, and then to the tv with another HDMI cable.