Your porblem sounds like a harmonic imbalance and that usually signifies an out of balance or out of round tire. It could however also be a warped (not bent so hard or immpossible to spot when the wheel is staionary) rim.
The best way to check is to have the tires balanced by a GOOD shop that will pay attention to things like lateral and vertical runout of the wheel, as well as wheel weight positions etc etc.
Actually if you can find a shop that has one you can get everything measured accurately with a HUNTER GSP 9700 Anti-Vibration machine.
http://www.hunter.com/pub/product/balancer/4159T/index.htm
http://www.gsp9700.com/pub/news/4412T.htm