I have had this steering wheel wobble that happens intermediately now on two vehicles- and both times it turned out to be the same thing. Not saying it is the only thing that can cause it, but for me at least in both cases it has been the same thing. Basically each time one rim had a slight buckle (which the eye cannot see- very slight). And it only on one side of the rim. When you hit an object with a wheel, your wheel will never hit it square, never mind that the object (like a pothole) itself will not be a perfect square. This results in the rim getting buckled, but depending on how it struck the object, one side of the width will be more buckled, the other side may not be buckled at all. Now as you drive on a highway, there is a camber for water run off. And the camber shifts from left to right. As the weight of the vehicle is shifted from left to right, it begins to get a steering wobble depending if the weight is placed on the buckled side of the rim, and the steering wobble becomes less as the weigh shifts to the round part of the rim as the camber changes. It also only does this at highway speed because it is a very slight buckle, you never notice it in town driving. It does not matter how many times the wheels are balanced (will always come out as perfect), the balancing machine cannot simulate weight being put on the rim shifting from side to side. The only way I have found how to fix this is to to swap each wheel one at a time with the spare (front or back wheels I've the same symptoms). You will notice straight away when the offending wheel is found.