Why it's flying? Because it's fast!
now more serious: 20/25 knots is quite a lot of wind for a freeracedesign as a lightning. In able to get a more stable platform follow the tuning tips given in this topic first. If they won't help you, pick a more downforce racing sail.
I used to own a futura 101 (2008) and it was really great in 20-25 knots with an NP RS4/S 6.2 sail !