Normally the nav module produces the layout on the screen. If nav unit drops out the video module (versions with TV option) produces a substitute more robust layout. So sounds like the nav module is on it's last legs. You can test how it behaves without the map CD/DVD inserted especially if it's burned, not original disc.

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey