First of all, I want to thank you for your time in reading this. I'm a relative newbie to APM, but I do follow directions pretty well, at least I believe I have.
I have a new heavy octo with APM2 (2.5.3) that I have stabilized pretty well, loiter works pretty decently as well, with the exception of altitude. I can be in a steady hover at exactly half throttle and when I switch to loiter, the craft will either climb pretty steadily unless I drop the throttle, or sink pretty steadily unless I increase it. This is at a starting altitude of approximately 3 meters. I DO have a sonar, which evidently is working as I have very granular readouts in the Mission Planner for altitude (past the height of the landing gear, checked that first), even without a GPS lock.
Even once I arrest the altitude change, it never stays there, it will still deviate by several meters until I correct it again. This is the same in ALT_HOLD mode or LOITER. I haven't tried higher altitudes yet.
I have attached the two most recent logs for your review. I would appreciate any feedback. Thanks again.