Initial wobbles, then nothing...

[Audidriver]

Hi there.
Just wondering if anyone might be able to help me out with a problem?
So the Hex is a Tarot 680 Iron Man, the FC is a Naza M2.
Motors are 800Kv Sunnysky, ESCs are 30A AFROs.
The problem is, on take-off the hex immediately starts oscillating in just about all directions, as though the Flight Controller isn't even there.
This will continue for about 20 seconds (almost consistently), at which point the whole thing settles down within a second and then flies completely normally, until the batteries are flat and then the same thing will happen again after a new set of batteries and a subsequent power up.
It occurs to me that this could be a gain related problem, but why should it suddenly settle down? Does the Naza have some sort of cancellation, or gain self-tuning which might be going on?
The LED sequence is always correct: -, wait until there are no reds, rapidly flashing green to signal home position lock, and so on.
The problem is the same in both GPS and ATTI modes.
Batteries are a pair of 16000mAh Multistars, which give around 18 minutes flight time.
All up weight is about 4400 grams. Props are 12x6's.
I also have a pair of 8000mAh Graphines, which give about 12 minutes but exhibit the same initial problem.
It occurred to me that the heavier 16000's might need different gain settings, but it seems odd that same problem exists with the different batteries.
Basic gains are: - Pitch 120, Roll 120, Yaw 100, Altitude 110.
Attitude gains are Pitch 130, Roll 130.
If I get some decent weather, I will post some video.
Any help would be very welcome thanks in advance.
Great web page BTW.

[ched]

I assume you have tried calibrating (Naza dance), checked FC is not lose, all props in reasonable balance, nothing lose?
You haven';t got a gimbal doing it';s setup routing while Naza is going through it';s startup?

Doesn';t sound like gains as, as far as I am aware they are used as soon as motors start spinning. I don';t believe that have any ';learning'; capabilities.

Does sound weird, only thing I can really think of is calibrate everything you can in DJI Assistant and the compass dance.