If it's only happening on specific channels, it may be either the way the specific channels are being broadcast, or an issue with the cable/sat box. In many cases, calling your cable/sat provider and having them reset, refresh or update the cable/sat box can clear up these problems.
You noted specific channels, so that may be fix the issue. Try that, and keep me posted.
Hi Samsung Support,
Samsung LCD Model: LA40A750
OS version: 2008/11/12_001010
I've this TV for abit over 1 year now and don't have any problems with it until now. The screen shrinks after a few seconds with a dark band which is about 1/5 the size of the screen on the right. I have to change channel and return and it will be ok for a few seconds before the dark band appears again. It will be ok after I keep doing this for a number of times. I also notice that this only happen with OneHD, 7HD, 9HD channels. What could have cause this problem?