Jille brings up a very good point.
Often, when a fin let's go (spins out due to over pressure cavitation) it very often is
due to either diminishing wind, change in wind direction, or change in board speed.
A fin can withstand only so much pressure at any give water speed past it.
So. if the speed drops, but the pressure does not, it spins out.
If the wind drops, often we fall back a bit as we are not longer supported as well
but the harness and rig. This results in us putting more pressure on the fin, at a time
when board speed is diminishing. So the fin spins out.
Overall, it could be a technique issue or it could be a fin size/quality issue.
Have you wet sanded this fin to remove the logos and painted on printing?
Is the leading or trailing edge damaged.
You can try a larger fin but on the Futura 155 with an 8.5 m2 rig (in 25 mph/21.7 knots ???) I would think you would want to use a smaller sail and perhaps a smaller fin.
Are you sure that your board is not "tail walking" rather than spinning out?