My FC40 started drifting to the rear and right on takeoff...after about a minute of jockeying it around, it smooths out and flies normally. Assuming that I had a compass issue, I hooked it up and sure enough, my compass reading was varying between 3900-4400. I did a full re calibration of the IMU, no change......I tried to de-gauss the compass as per instruction and it seems to have made it worse, the readings now varying between 9800 and 11000. I happened to have a brand new compass so I put it on and got the same readings as the old compass which leads me to believe it's not the compass. I'm stumped.....anyone have an idea of what's causing the problem and how to fix it? Any help will be greatly appreciated