Are you doing capture/playback through Myth or are you talking about watching live tv through the capture card in LinuxMCE outside of Myth?
I am not having that issue, but I am only using the tuner for my pvr-150 (and HDHomeRun), no s-video capture and watching everything through Myth. What I have noticed that may help is since upgrading to 810 the new version of Myth automatically controls zoom on the fly. For me it works great even if it sometimes zooms a 4:3 picture cutting off the top/bottom to fill my 16:9 screen.
I believe I read that Thom is working on using MythTV's playback for TV recordings in the LinuxMCE video section. If so, you might want to check Myth settings or even search the Myth forums.