I have a TBS Discovery frame running APM 2.6, I can't remember off the top of my head, but I think 3.2 is the firmware. The power system is a 4s powering 750KV motors with 9 x 4.5 props and Afro ESC's. After a good deal of work, I was able to get it tuned to the point of flying it confidently in Stabilize mode. There is still a little of oscillation in it but I have flown hours on it with out fault other than user error. It should be mentioned that this build has significant power, maybe even too much.
My problem is that, when switching to any modes that are self controlled, such as Alt Hold or Loiter, it immediately bounces up and down quickly and then rolls over to the left, resulting in a crash. I have not been able to figure out what the problem is, I don't have much experience with APM. I checked the vibrations yesterday as the board is mounted directly to the frame with 3M double sided tape that has about 1mm of foam in it. They all seem with in reasonable rates while hovering. The first log is this hover (2015-08-07). Today after work, I tried putting it into Alt Hold mode and, like every time, crashed. Wind was about 10 mi/hr, not unreasonable for this frame. The log of this crash has also been uploaded as log "2015-11-05"
If any one on here could help me out, that would be greatly appreciated.
I don't have a 2.6 (still on 2.0)
Still, I'd suspect barometer first if altitude goes haywire in barometer-dependent modes (alt-hold and loiter).
Is the barometer covered in (breathable) foam and shielded from direct sunlight?
(I'm not even sure off-hand, but is barometer health included in the pre-arm checks that you can activate to prevent arming/take-off if anything's amiss? Do you have the checks enabled?)