Satnav got a mind if it's own?

I have a 2004 E320 Cdi. the Satnav works when it wants to. I've got a 2009 version maps and have been using it since. It's not scratched. A message saying " can't read DVD... Please check " I've checked everything, but can't figure it out. Anyone else with the same problem? I've had some low profile tyre put on, could it be the rough ride causing this problem. I'm at my wits end. HELP. Thanks