mythtv 0.26 pip flickers and shows mixtures of unrelated frames. A patch is available upstream but not yet applied. This patch fixes the problem for me. Please apply the patch from http://code.mythtv.org/trac/ticket/10878 until upstream applies it.
I'm reluctant to incorporate patches long-term. I am tracking fixes/0.26, and if they merge the patch there I'll certainly have it in portage fairly quickly (I've been doing bumps about monthly but would be amenable to doing one on-demand if it were merged upstream). The ebuild does support user patches - if you place the patch in /etc/portage/patches/media-tv/mythtv/ it will be applied when you emerge mythtv. I'm using this for a patch for a transcoding issue that has been bugging me for months but for which I've yet to find a proper fix suitable for upstream.