My old 8300HD broke (No Audio) and Time warner replaced with 8300HDC. The problem I have is that I have a GEFEN 2x2 HDMI switch/Splitter. When the signal changes from 480 to 1080 and vice versa, the switcher takes 10 - 15 seconds to pur the video onto the screen. The old box kept the signal at a constant 1080i no matter if it is HD or not. About 1/3 of the time when I switch from HD to non HD source the audio or the video gets lost. Probably due to the security features built into HDMI.
I think you got set it to upconvert all signals to 1080i. Its under the settings, more setttings button. It probably says HDMI right now and it needs to be upconvert 1 or upconvert3, i can not remember which is 720p and which is 1080i