A couple of other app settings that I still have turned off that may be creating problems for you, if you are still having problems. I just haven't retested them yet to revet them. One is keeping VPS off (no adverse stabilty outdoors for me with it off) and turning off video caching (not sure what it is supposed to do, if on, but completely unnecessary), which keeps the bandwidth clean between the P3 and the transmitter and tablet.
As I recommended above, you must first recalibrate the IMU.
Then recalibrate the compass.
Then launch from a metal free area.
It works. If it isn't working, something else is likely going on that is unrelated to the FW update. The process, painful as it has been, has made a believer out of me!
I was already running with the video caching off. VPS off doesn't make a difference. However, VPS on made it amazingly stable before the 1.2.6 update. I can't imagine how it's *not* the firmware update since it went from being essentially a floating tripod before, to being completely unstable after.
I've calibrated the IMU and compass several times. Double checked the level of my table at right angles to the edges, and always careful to make sure I have no metal/RF like my phone nearby when I do the compass. None of that makes a difference for me. E.g.: