I will switch to ulog.
Flying with 1.5.5 stable release.
This fly away occurs repeatedly in all flights at exact time (nearly 2:30 minutes).
The problem is, this does occur only in HOLD mode after takeoff. If i switch to POS CTRL mode, it doesn't fly away for nearly 11 minutes.
The interesting thing is, when i switch back to 1.6.5 stable release, this does not occur in hold mode. (if the imu was problematic, this will occur in 1.6.5 also)