Originally posted by JDUB
I have the same problem. Sometimes the car is smooth, some times it ain't. If it is a balance problem it would be consistent. I suspect a worn bushing in the suspension, differential or trans. It the rubber is worn the component may shift into a position that causes vibration sometimes and then works its way back to smooth. Sounds simple but finding the real cause isn't.
How about the idler arm or the wheel bearings? I had a shimmy around this speed -- maybe a little higher. That seems to have taken care of the problem for me.