It's the fin!
However, I have made a few speed runs on my iS 111 and hit 30 knots with the 38 Drake fin with no trouble, but the water was very flat with 5cm chop at the biggest. I am guessing that you are in choppy water with 10-15 cm chop or larger. Any pressure on the back foot while going over chop with air getting under the board and you will spin out. Better fins don't do that.
I have a tectonics tomcat 42 fin that doesn't spin out. I use it with my 8.4, 7.6 and 6.6. It's a little big for the 6.6, but I prefer it over the Drake 38 without question.
Same deal with the Drake 44, but it's a bit better because of it's size. However, when it does go out, it's almost impossible to pull back, just like the 38.