Weird Steering Question

I just bought a 2002 exl odyssey and there are some things i didnt know if they are normal for this vehicle. When i am driving over a rough surface it seems that the steering wheel moves back and forth on its own. I assume that these are hydraulic assist power steering systems but dont know for sure. Nothing seems loose but it just feels as if a ghost is helping you steer. Is this normal? I also seem to get a vibration around 75 miles per hour on steering wheel only. I figured i would get it aligned and tires balance before getting to worried about the vib problem. Let me know if your steering feels the same?

Thanks
Sidman

have your tires rebalanced and rotated and alignment checked and because you might have bad alignment from hitting curb or potholes that can throw of your alignment too and if tire isnt balanced then the tire will tend to wobble and as you go faster it might get better or worse.. but if it wobbles when you brake then you have other problems