You're flying in Stabilize, which is a mode that does not use GPS.
Try to do the following:
1- Go to a wide area
2- Calibrate the compass and check that the north is really north
3- Put in your control the flight modes: Altitude hold and Loiter
4- Fly in Altitude hold, change to loiter and check if the dron stays in the same place
5- If there is a problem, return to Altitude hold, and land
Loiter: (Loiter Mode automatically attempts to maintain the current location, heading and altitude.) The pilot may fly the copter in Loiter mode as if it were in a manual flight mode but when the sticks are released, the vehicle will slow to a stop and hold position.)
6- If there is any problem let me know and send me the log.
Sorry for the inconvenience