TomTom software problem

I have XDA IIi and also purchased TOM TOM software off the ebay, which i thought was the real deal.., but I am having problems. the software is loaded and I have the tom tom receiver, but once having enter the destination, and started of on the route the tom tom switchs off., and I am having to re-enter the details again..!!, can anyone tell me whats wrong.?

Is it the software that is switching of or the PDA?. It could be that you need to set the 'turn off' when on battery setting to ON.
I have mine set to remain ON when on external power as I have a cradle connected to the lighter socket in the car.