I've had my N9 for a few weeks, and of course it's great

Except for the GPS fix - the fix is obtained quickly but it's unreliable. When I navigate I frequently get a "GPS signal lost" message, and it quite often takes a few minutes until the signal is restored.
I tried a speedometer app and found a similar experience, worked great and then dies for a few minutes at a time.

I've completely reflashed the device, that seems to be common advice. Has anyone had similar happen? I might have to take it into Nokia Care but I don't really want to be without it while they investigate...