Acro brutally banks backwards

Hi everyone,
I 3d printed and build a mini quad with an APM 2.6 controller. I tuned it manually, but still have a big problem in acro mode : i’ll throttle up to hover, then push forward to go forward (logic). The quad hovers and tilts, then goes forward for about 10 meters before banking backwards suddenly. It’s totally impossible to counter… Did anyone have the same kind of problem before ?
Could this be caused by a too low I gain ?
Thanks for your help !

Here I have some logs showing what I mean.
When observing the Attitude control, we can see that after a little bit of fast forward flight, the quad asks for a violent backwards fly… But I can’t find why it asks for that… Help !

2016-06-16 18-08-02.log (736.8 KB)