at those speeds your problem has more to do with aerodynamics. if you wish for your car to be more stable and predictable at those speeds you'll need to fit a functional front spoiler, sideskirts and rear diffuser. these will reduce the amount of air going under your car and will help suck it to the ground a little.
if you wish the car to be more stable you can change the suspension for a lower and stiffer setup which will reduce body roll and you could upgrade those horrible general tyres for something which actually has some grip.
please note that this will just make it more stable but it will still be as susseptable to cross winds because you are not altering the shape of the car from the sides (where the wind hits it). this has more to do with the actual shape & design of the car.
remember, all the stuff i have told you is for aerodynamics and vehicle dynamics, not ricing.