Aircraft Weaving / oscillating in auto mode only after ~5 mins in flight

Aircraft weaves between turnpoints, often quite irratically.

Flight starts and first 20 turnpoints are perfect then the weaving starts.

Arduplane 4.0.9
Omnibus FC
Simple glider with 2 servo wing.
No compass, just BN280 gps

FBWA , loiter seems great as do all the other modes, just navigational modes.