At 92 kg on a 6.5, fully powered, I am guessing that the board is too big for the conditions. A smaller fin would help with control in high wind conditions, but 50 cm isn't large for a board that's 80.5 cm wide in winds from 12 - 20 knots. Over 20, you should be on a smaller board.
When you are on the 6.5, what speed is the wind? Do you have a smaller board?