Navman F20 fault

I am having a problem with my Navman F20, a problem which the brains ? at Navman don't seem able to sort out, hopefully someone here can help.

Situation 1 : Switch on, enter a post code I am going to, wait for the device to pickup the satellites. I then get a verbal first instruction and the device works okay.

Situation 2: Switch on, enter a post code I am going to, wait for the device to pick up the satellites. Then no verbal instruction, I cannot scroll down through the whole route and the screen keeps going blank with the message across the bottom saying "back on track". Yet I have not moved.

Thanks.