Please forgive
this cross-post from the XBMC forums.
I am having a similar problem.
I am in Australia - all DVB channels broadcast are 16:9 whether they are HD or SD. All DVB channels here are MPEG2.
See my system specs below. When I first start watching Live TV the video is stretched to full screen (4:3) whether it is an SD or HD channel. After changing channel the picture changes to 16:9 letterboxed (correctly). However, if I have Settings/Video/Playback/Display 4:3 videos as "Normal", when the Live TV is displayed letterboxed, it also crops part of the left and right side of the picture - cutting out part of active vision. If I set Settings/Video/Playback/Display 4:3 videos as "Strech 16:9", it fixes this left/right cropping issue. But I still have to do at least one channel change to fix the full-screen stretch issue.
I also occasionally get the black screen issue. I find pressing "Stop" (x on the keyboard) and then re-watching TV fixes this.
I have started using TVHeadend with XBMC after giving up on MythTV Live TV due to stability issues. TVHeadend seems more stable. But MythTV did not have these aspect ratio / cropping / black screen issues that TVHeadend seems to have.
TV: 4:3 CRT - composite input
XBMC Platform: Raspberry Pi 512MB RAM
OS: Openelec devel-20130107142611-r12816
OS Build: OpenELEC git: Commit:29d4e56
XBMC Version:12.0-RC3 Git: c47e925 Compiled 7 Jan 2013
TV Backend: TVHeadend 3.3.402~ga958415
Tvheadend XBMC Client: v1.6.10