Probably some combination of all of the above issues.
Move the mast base forward a little to drop the nose.
Try different stances.
Be sure you are leaning away from the pull of the sail, not back toward
the rear of the board.
36 cm in 20-25 knots on a 101 liter board sounds far to big to me.
28-32 cm and a bit curvy (not a slalom race fin) would probably help alot.
Hope this helps,