Jerry, I was using Maps (not navigation) this weekend. It, like you said got location very fast and kept my position updated. However if I let the screen sleep when I went back in the map would be blank. I then had to hit the power button and unlock screen, once back in I'd get an FC from maps. This happened every time the screen locked. If I got it when the screen first went black but hadn't locked yet it was fine.
As far as I remember I had 3G the entire time.