had the exact same problem, stock 36 fin and frequent spinouts. wind 15-20 knots, i101+7.7 , 73 kg.
this was happenig at the spot whit nice , long waves (0.5-1m)where you could let your speed go- on another spot , with lots of 0.5m chop it wasn't happenig (maybe because i didn't try to go full speed).
paired with 42 stock fin, everything felt very nice (tried it when the wind dropped to 15 knots or less ) and fast although i would suspect that one should be slower with such a big fin.
i think i read somewhere that sanding the fin helps but i wouldn't know where to start since the finish of the fin is without the smallest scratch/dent/defect..