To get less stiffness on its bad days should i set it up as a twin fin?
I don't think that's the answer to your problem. Aside from changing your approach to the wave on those days you can't get your board on a rail, you can try a fin with less rake. Those AM fins look like they have a lot of sweep, which draws out turns, but maintains speed through them. A more upright, pivoty fin could be the answer.
But you might try moving your back foot back. Chances are your finding it hard to initiate a turn when the waves are letting you go faster. The faster you go, the flatter those wider boards like to stay. Stepping back with your back foot can help.