Why does my HDTV keep switching from widescreen to full screen?

It's actually my dad's new tv, but it's quite annoying. I live a couple hours away, so I can't fiddle with and try to fix it. Anyway, the problem is that it will be in letterbox format with the black bars on top and on the bottom. I know this depends on the aspect ratio, but then it will also go to full screen and... show more It's actually my dad's new tv, but it's quite annoying. I live a couple hours away, so I can't fiddle with and try to fix it. Anyway, the problem is that it will be in letterbox format with the black bars on top and on the bottom. I know this depends on the aspect ratio, but then it will also go to full screen and fill the entire screen vertically, removing the letterbox. It has done this on many many occasions to the point where my dad is ready to return it.

It switches during the same program (like almost every 45 seconds or so it makes the switch) so I know that's not the issue. It also does this during movies being played on their DVD player. Is the TV trying to find the best fit and can't decide? Is there some setting on the TV to stop this from happening???
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