Make sure the GPS has settled down before you ride.
Noticed this much as a problem on my iphone 4. the iphone 5 seems to be dead on and settle out very quickly.
Also, I did have to restore my iphone 4 once. The GPS was all whacky. Took it to the apple store and the genius there ran a diagnostic. A part of the system was screwed up and running slow. When I restored and reset it all, that fixed it and the GPS worked much better.
J.