I'm interested to know if this problem was resolved? My new inspire pro has similar problem - seems much more unstable than my old Inspire 1 and also my P3P, and also demonstrates this random dropping in altitude. I watched it drop 3-4 metres without input from me, and then looking at the display on the screen, saw that the altitude was not updating correctly. I eventually landed and it showed 7m altitude on landing, and then a second later it was back to 0m.
I'm pretty sure my old inspire was not like this, and as it stands I don't really have much confidence that this is working correctly. I am runnign the latest firmware (1.8.something) and gains all at 100. The wind is mild outside (as good as it ever is). What should I do? I haven't done an advanced IMU calibration since receiving it, but then I don't think I ever did on my last one, or my P3P and it was fine.
Will try running one later, to see how that goes