My 1.1.1 drifts to the left when flying straight forward. Flying backwards it drifts to the right. We have tried IMU calibration, compass calibration, and stick calibration. Yesterday I tried something different. Took off and set out flying straight, and it was drifting left as usual. Then, while still flying I switched it to atti mode and it suddenly went straight. Switched it back to GPS mode and it immediately began veering left again. Any ideas on this? It's clearly GPS related somehow. I could get a video of it doing this if it would help.