Upgraded to a P3Pro from a P1 this week. Updated firmware on Phantom, camera, and remote, calibrated compass, and made a very uneventful first flight. A few hours later, same location, began a second flight using "auto takeoff",and the craft rose a few feet, then began flying circles on its own. I was able to fly it up to avoid trees, but the circling continued at speeds up to about 25mph according to the flight data, with no control input from me whatsoever. RTH and auto-land commands went unheeded. After about 3 minutes, I was unable even to maintain altitude and the circles began to tighten, and the Phantom crashed into a tree then fell to the ground. These machines are built tough because there is only superficial damage apparent. I checked the IMU calibration post-crash, and though it checked OK, I did another calibration. This is my first post on this forum, and I'm hopeful the members here might shed some light on what could have caused this major malfunction and what I should do in order to avoid a repeat of this potentially disastrous scenario.