Check tire pressure, balance and wheel alignment, when any of these are off it can cause problems like this, if any of the above don't fix the issue then it could be the suspension, but that's unlikely.

Also, last but not the least, heavy side winds can do this to a car, but usually it's one way, unless it's cross wind, again, unlikely.

 

How many miles on it? If like most, and it's beyond 150k or so, it could be suspension related. Or nothing.

Is the highway heavily rain-grooved? Some tires are more prone to "tramlining" than others due to tread design, carcass construction, stuff like that.

If that's not it, it could be struts, tierod ends, ball joints, stuff like that. Check the rear rubber bushings on the front lower control arms. Those quite commonly are torn.

 

If its not tramming, tell mother nature to quit blowing in the wind.

 

grab each tire in the 3 and 9 o'clock positions and try to turn each wheel. obviously the back wheels shouldnt move at all so any movement would be bad. if you pull hard enough on the front, they may turn but with great difficulty and with even pressure. there shouldnt be an loose feeling about it.

 

i've had the alignment and wheel balanced when i put on new wheels 4/5 months ago. the tire pressure's right and there's no cross wind, well, the trees are not moving on the side indicating there's no wind.
the car has only about 110k miles on it.
i have my rear wheels cam-in a little bit but that shouldnt be the problem. my guess is if thats the cause the problem should appear a long time ago.
well, if it's suspension then it's time to lower it a bit!

 

ill try it again today and if the problem still exists then im taking it to the wheel shop for a check up.