This happens when the gps is weak. I've seen it when I go under a canopy of trees. I've also seen it when I went inside a building once. Basically when the satellites are blocked.
Are you near tall buildings or lots of trees when this happens? Another thing might be how you're carrying the phone. When I carry it in my hand or in an arm band with the top of the phone pointed upward it seems to work really well. I carried it in my pocket a couple times and the gps tracking didn't seem to be as good.
