I have a board similar shape and size to the one in question and was having the same issue.....First.... the board is designed to be loose with the super pulled in tail so in better surf it's gonna be different unless you're used to such boards. I ride a higher volume board and like the pulled in tails to compensate for more foam. I started off with IFT M5s in the front and used several rears with no problem in average surf. First time I got into bigger, hollow surf I slid out and ate **** cause the rear was so loose........I used more fin and the board works great on big days now. I put IFT M7s in the front and standard M5s in the rear. I'm 6'1 205 so I dont know if this will help but it's worth a try if you like the board and want to keep it.
If this doesnt work for you one sure fire way to calm down a loose board is add some weight to the rear by adding a 3/4 patch to it. I did this to an extremely loose board and it stablized the rear making it an excellent ride for bigger days.