I am having a problem with the display brightness on my Tomtom 730. It will start out ok, and then for no reason it will darken up and we cant see the screen. If I turn it off and restart it sometimes it will be ok for a while but sometimes it will revert back to a dark screen. Other times it seems to be ok for days. I have checked all the brightness preferences etc and they all seem ok. What could the problem be?