I'd second checking and adjusting your alignment. I believe the stock factory setting had a little toe out. 225 with lower profiles on 16" wheels, right? They will tramline and have an unstable, darty feel overall on stock alignment settings, which were for much narrower and taller tires. I had the same issue on 225/50-15, very darty, solved at zero toe. Some tire designs are more sensitive than others as well. You can have it checked out and set to zero toe or a little toe in for even more stability. Reducing caster will also lighten the steering a bit (but may get a rub on the front).