This is not the usual vibration issue. This vibration comes and goes, regular like. I just rotated my tires and the backs are now on the front with a side-to-side switch. The fronts went straight back. Now, at highways speeds, a vibration starts, reaching a peak, and then disappears, only to come back again, over and over. I feel it in my butt and the steering wheel shakes. Now how can this make sense? I've had it happen on various vehicles over the years every now and then and I'm just wondering why it happens? Surely I'm not the only person this has ever happened to. Shouldn't the vibration just be constant if a tire is out of balance? All I know for sure is that it's the front tires. Explanations?