I had a similar experience with both the Isonic 111 and Isonic 96 yesterday.
Sailing in fairly bumpy knee high chop with an '09 6.5 m2 Sailworks Retro.
Have you checked the surface of the Drake fin to see if there's any raised
areas in the graphics.
I've had some success rubbing out the graphics to get the surfaces of the fins
smoothed up, but both of the Isonic fins I tried yesterday had already been
Both of them spun out pretty easily.
The 46 cm on the Isonic 111 let loose, but was fairly easy to get back without losing too much speed.
The 34 cm on the Isonic 96 would let go big time, tail of the board went downwind at least 30 deg. and you could not pull it back without almost completely stopping and changing course way downwind to reattach the flow.
So, I agree, there are some issues with the supplied Drake Fins on the Isonics.