check your tire balance and tire condition, ball joints struts mounts ty rod ends.Most of that stuff should have been checked when the car was aligned! Also wheel bearings,You could have a broken belt in one of your tires,under inflation,over inflation

 

what he said;

worn tie rod ends can allow the wheels to turn left and right a tiny bit, and at road speed, could cause them to flop back and forth quickly.

my base model did the same thing

also check the ball joints too; of those are worn, they could allow the whole strut/spindle/hub assembly to move about and the caster/camber will be "dynamic" and change all the time and the car will wander.

my base model did the same thing.

it was nerve racking to drive on anything but dry pavement. the car would just go left or right all by itself even if i held the wheel rock steady... its WAY scary trying to drive a 2500 lb rwd car in the snow w/ bald tires in the rear w/ a car that goes where it wants.

 

I had this happen and it ended up being a separated tire!

 

have you had your tires balanced lately? it sounds to me like you haven't, if it only happens over 100 km/h (60 m/h for you south of the border), do that and you should be good to go. good luck.