GPS working erratically in iPhone 6.

The GPS in this iPhone is acting very oddly; it's not updating in real-time. Drive down the street and the GPS doesn't register any motion (although it does seem to register turning left and right; the arrow turns in place without moving). Then all of a sudden it will update, jumping ahead all at once. It is effectively useless for real-time navigation.

Things tried include resetting Location Services, changing map apps (Apple Maps vs Google Maps), and factory restore/clean setup. The phone has no other issues. Cellular signal strength is strong, and wifi access (when in range of an access point) is reliable. It's definitely something to do with the phone itself; the same SIM card (Red Pocket) in an iPhone 5c works perfectly.