Stationary TB effect = Hula-Hooping

Here’s a weird phenomenon I experienced on my (overweight) TRex500 starting back during original tuning, spring 2014. I slowly eliminated the problem by juggling various parameters, not sure exactly what did the trick. I mention all this now because after a crash and rebuild, I’m seeing the problem again but not nearly as seriously as back in 2014. It would be great to get some expert opinion on what causes the problem and what parameters can fix it.

The problem is this: AltHold works fine. Then switching into Loiter the heli rocks to one or the other direction and then commences a stationary roll/pitch oscillation as if it were trying to swing a hula-hoop! It remains firmly in place while rocking, but if the effect is very pronounced as at the beginning of my tuning days, it does begin to go wild and exiting from Loiter is recommended;>)

The effect is surely related to this graph from the log, this one was at the beginning, with serious oscillation:

Upon hitting Loiter, you can see first Pitch starting to oscillate, followed closely out of phase by roll. Easy to understand how this translates to the hula-hooping, but just what is causing the FC to emit these commands remains a mystery.

With further tuning of the parameters I reduced the oscillation from ±18 to ±4 and later I think even lower as I cured the problem to almost zero effect. Although it still appeared occasionally just at the beginning of Loiter, then died away.

I am 99 44/100ths % sure the compass has nothing to do with this, as I did many recals, movings to different locations, etc., even a manual recalibration of offsets that gave quite a bit better accuracy than the MP auto-routine. The compass is now out 1/3 the way on the tail boom far from any interference. GPS is closer in. Vibrations also don’t seem to be the problem, they are within limits.

I’d much appreciate any informed opinion, since I am still somewhat in the dark as to the causes and cures, having arrived at the latter mainly by chance! By the way - I did try FW 3.2 on the APM, but went back to 3.1.5