These steps have been known to be quite effective in fixing this issue, though there's no theoritical or logical reason behind why these steps work.
1. Reset the device when problem occurs.
2. Charge the TomTom unit completely.
3. Don’t turn it on immediately, and place the unit somewhere in the open (like on top of your car).
4. After you wait a while, turn it on and wait for it to lock onto a satellite; hopefully you'll receive maximum signal.