I know this thread is a couple of months old but I have some information about this issue, though not a permanent solution. I have the same setup, though my receiver may be the 373, can't remember and can't check at this moment. The issue seems to be with the on screen display of the yamaha receiver which I think is displaying in HD while SD content is showing causing the switch. If you haven't already, you'll notice that if you bring up the yamaha menu's the same problem occurs as changing the volume, in other words as I said before, it seems to be with on screen display from the yamaha receiver. A partial fix that I've found is turn off the motorola dvr, hit the menu key and wait a few seconds and menu will come up that lets you change some of the dvr's video settings. Under this menu change the non-hd content setting to "Stretch", hit the menu key again to exit and power your motorola dvr back up and the problem should be gone. The problem now is the setting doesn't seem to stick, for example turning off your yamaha receiver will cause the motorola dvr to revert the settings through I guess some auto detection process, don't know. I saw another thread somewhere with a similar problem that looks like it may have been solved by switching from HDMI to component video, haven't tested this yet personally.