I don't intend to have a monitor attached in the long term but unfortunately I still have problems with MythTv not displaying the video at the proper size and shape in the TV Monitor.
Buried in the myth setup are options to change the X Y position of the playback. Try tweaking those.
Once again, mplayer works flawlessly from the command line giving a nice full screen playback from the tuner but Myth does not display properly in "WatchTV" mode. The GUI looks fine but tuner playback is terrible.
If the playback is terrible, that's the OSS nv driver. Use the nvidia binary driver instead. FC has prebuilt rpm's that track the kernel, I think it's from livna. Other distros will vary.
You still need to rule out if it's a recording or playback problem. Try recording a 15-60 sec clip and email me the .NUV file and I'll tell you.