I have noticed the same problem. I cannot detect a pattern. I have seen lip-synch problems on DVDs, on HD channels and on SD channels, but never consistently and to varying degrees. There have been other mentions of this on this forum. You might find out something by doing a search. I have the service people coming out for something else and have asked them to check this out. That's to take place Saturday, September 27. I'll let you know what, if anything, happens.
(I have an HLN 5065W watching Cox Cable through a SA 3100 box. My DVD is a Samsung 931 with a DVI connection.)