It cannot drift and flip at the same time, because a quad flips if motor layout/prop orientation is wrong on takeoff or acc/gyro calibrated wrongly etc. Every time you reset/flash your board, settings are lost so make sure you select proper motor layout, assign switch for auto level etc.
If its drifting you need to tune, check the thread for more info.
http://www.rcindia.org/multirotors/quad-tends-to-drift-forward/And yes decreasing the throttle below a certain point may suddenly bring your quad down, because your quad attains enough thrust say at 40% of throttle and it would begin to hover, and if you decrease it below 40% it will come down. Now you can hover at 5' or 20" at 40% throttle and the moment you decrease throttle it will come down. Even a gust of wind can bring your quad down if you were hovering at the threshold.
For this you need to set a throttle curve in your radio or dr/expo which Im not so good at. Google it to find more info.
What you can do is
1- reset the board
2- perform acc calibration on a flat surface
3- choose motor layout and check prop orientaion
3- assign auxiliary switch for auto level.
4- select how you wish to arm the board
After completing above steps, try to fly the quad in auto level and tune for drifting. I must add that your quad will never hover or stay in a spot, there will be some amount of drift but very little.
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