I've got a DCT3412 here from Comcast and have noticed the exact same thing, but, it goes away.
Example: I had snagged The Andromeda Strain off UHD. Watched some of it output at 1080i to my RPTV and all was well.
When I moved the signal to my 720p it began to drop frames like crazy. Note, this isn't a simple "720p handles motion better"....this was chunks of movie were suddenly going missing, as much as a second at a time. Switching output to 720p fixed it up.
But then, when I switched back to 1080i.....it was fine then, too.....so who knows...I wonder if the HD was busy doing something in the background that made it studder, or if every now and then it just needs to be turned off and on again. <shrug>
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