There are reports that some receiver boxes and devices have issues. Since you have ARC working I think I would not try new HDMI cables but would try to see if the receiver box has some setting to nail down the video to 1080p. Try it at 1080i and 720p for testing and if that sticks at less than 1080p then you call both box makers and ask what next.
Why didn't I write a word about firmware? Because HDMI firmware is in the HDMI chips (so far) and firmware updates don't usually help that. But do check firmware updates and keep that return option open as if it can't be fixed fast enough we want to let them keep the boxes.
I have a Samsung HT-E5550/XE Home Theater connected to a Samsung UE46ES7005. Right now it looks like this:
Fibre lan->Ethernet Switch->Digital Reciever Box-> HT-E5550/XE->UE46ES7005.
The HT5550 & UE46ES7005 along with the digital reciever are connected via HDMI cables. The HT5550 and UE46ES7005 are both connected to the HDMI (ARC) connectors so that sound can pass both ways between them.
My problem is that when I switch channel on the digital reciever box the TV will occasionally loose the picture where the picture would be "white noise" or just black or flick between white noise and black screen, but sound will remain other times the picture also in a green or yellow hue with the noise. To fix this I have to either power cycle the digital reciever, or HTE5550.
Now if we look at how everything is connected if I now bypass the HT-E5550 and connect directly from the digital reciever to the TV I have no issues the picture doesn't glitch out or anything and I can change channels however much I want without the TV loosing the signal. This leads me to believe that there is a problem with the Home Theatre system.