The penny dropped yesterday: the USB power supply on my bike does not have enough juice to keep my phone going continuously. It is proper motorcycle unit, but obviously not a very good one.
This has caused me to lose a lot of GPS tracks and to get lost a few times.
I will modify it when I get home, but its a problem right now.
I need to find out the operating current of the Lumia 920 phone and make sure the USB power supply can provide that and a bit more, so i can recharge the battery and used the GPS apps at the same time.