I bet that's your TV switching between resolutions - the menus are all 480i while your HD content is 720p or 1080i. Try switching the HD's video output format from "Native" to "720p fixed" or "1080i fixed".
(Apropos of this, I wish there was a setting that output 1080i content in 1080i and everything below in 720p)
(Apropos of this, I wish there was a setting that output 1080i content in 1080i and everything below in 720p)