It sounds like your vehicle is losing connection to the MkIII nav drive. When this happens, the video module takes over the system and provides the menus that you are seeing.
First thing I would check are all of the connections to the nav drive (note - never remove the connectors on a MkIII when the drive is active, it can permanently kill it). If everything is properly secure and connected then you may have a nav drive that is starting to go bad. You could try doing a downgrade, but you would have to step back to at least V21.1 (V22 and higher all take the MkIII to 3-1/63 and from the MkIII perspective are all the same disk).
The Nav faq -
http://www.xoutpost.com/showarticle-59.html
Sources for the OS -
http://www.xoutpost.com/showarticle-88.html
Jeff