Arducopter 3.1.5 slow wobble in V quad

Hello guys,

im trying to help a friend solve a strange slow wobble in loiter. this is with the RCTimer Spider quad running 5010 610KV motors and 12" CF props on WiiESCs.
it does happen in stabilize and ALTH as well, but less, and is significantly more apperant in loiter.
copter has been auto tuned and seems to fly well otherwise.
BTW - it happens much less when there is no wind - loiter with no wind seems very stable.

please see attached log.

In V-quads, the CoG/CoL are really important! Additionally, tuning is rather complicated because you probably have to tune roll and pitch independently. I had a similar problem with a similar quad and I finally trashed the frame plates and got F450 ones. Much more stable flight with thoseā€¦