My TomTom GO500 has taken to switching itself off randomly during a journey, or won't even switch on in the first place. Usually complaining of no battery, even though it's plugged in - using the cable and connector provided, into the cigarette lighter. Have tried alternative cable, portable battery pack, with and without the mount and the same thing happens. Any suggestions/advice, or do I just give it up and look for a new one? It's about 4-5 years old I think.