I am experiencing the same thing, although it does not shift automatically when playing back 480p MKVs. If I were to hit a button that makes a menu appear (audio or subtitle), then the display will shift to 4:3. Hitting stop, returning the previous menu and replaying will correct it.
With 720p MKVs, the shift happens at the moment of playback, It appears to be something wrong with the 16:9 Wide/Auto setting in the Setup menu. If I disable the Auto switch for 4:3 content, the 16:9 material will playback fine. Of course, then my 4:3 stuff will appear stretched. I suspect this will be an easy one for Oppo to correct.
No dropouts on Star Trek.
Originally Posted by vojin.stefanovic
I have reverted back to May firmware and the problem is gone.
I did not investigate the issue beyond establishing the fact that when I played 1:2... format MKVs form Synology NAS OPPO narrowed the picture by adding the black bars at both sides - like watching 3:4 material on 16:9 screen.
The other interesting thing is that I do not have Reset Factory Default under Device Setup. So I could not do it. This also applies to May firmware.