As somebody who is having a few issues with his Sat Nav at the moment too, I'd get 4-1/100 installed just so it's running the latest.

I've had times when the GPS has pitched me right in the Bristol Channel but it's sorted itself out after a while, but if yours has been going on for a while then it's probably something more fundamental. Agree with your idea about killing your power - like PCs it's amazing what can be resolved with a reboot!