Originally Posted by neilG

Get up to 35mph often, do you?

Seriously, isolate the problem by trying another wheel, another tire, etc. I've never seen a wheel present that kind of problem. Frames, yes. It could happen, so if you are certain it IS the wheel, make sure it's dished properly, the tire is not damaged somehow, and there isn't the OEM reflector still on it. Something that would cause it to be unbalanced at those speeds.