I have the same phone and let me give you a tip that I've noticed (you may have noticed as well); When you start strava, wait for the status at the top of the recording screen to read "GPS Signal OK"

Whats happened to me is I turn on GPS, hit the record button, pocket the phone and go. But the GPS can take a couple minutes or more to find a signal, so I was cutting off the first few miles of my ride. What I've found is that once it finds the signal, its pretty reliable in keeping the signal