isonic 117 wide
i have had this experience to.
Reason have been the harnesslines in the wrong place and to much downhaul on the sail..
i try to use a very upright stance on this board and this helps a lot to discover if you have a good trim.
It feels like the board react very much on smallest change in trim.
i tried a 9.5 m2 sail on the board and in to low wind but still planing i got to much back foot pressure.
i used a 47cm select s10 fin.
The select fin works great with my 8.7m2 sail but i have ordered a blackfins project type r 50 cm to match this sail.
I use a 45cm blackfins project type r with my 7.9 and this is works great to if the max wind is around 17-18 knots and flat water.
im 77 kg
yOu should check your trim because the board is really fun when you get it together.