Something I've noticed in my past 3 or 4 flights.. my P3P spins counter-clockwise very slowly. What causes this? Bad compass calibration? I've recalibrated the compass twice and it still does it. Maybe it needs another IMU calibration? Any ideas?
Skip the compass and IMU calibration. Connect to the Phantom with the Pilot app and enter settings (upper right of screen). There is a setting to recalibrate the sticks. Do this.
Drifting would most likely be IMU but turning would most probably be the Sticks calibration as has been said. Doing both should correct the issue. Use a level on the quad before doing the IMU. Check it front to back and left to right ir right to left and back to front.... Makes no difference as long as it is completely level. Use sheets of paper or if your rich $100 dollar bills under the feet to get her level. It's that important. Enjoy and good flying.