Sounds like there was an HDMI Sync issue. When you ran a new cable did you have both the TV and DVR turned OFF or just plugged the new cable into each unit? Did you try power cycling both units before running the new cable trying the Samsung TV?

The Samsung TV would have likely sent a different/new Sync command to the DVR in which the DVR acknowledged and provided picture. Moving back to the Sony which would have then sent a different/new sync command than the Samsung would have re-initiated the video signal to it.