Does the vibration appear related to road speed or engine speed? If road speed, the intermittant nature speaks of something related to wheels/tires/brakes. As you turn, go over bumps, etc, the dynamic relationship between the front wheels changes. If the imbalance phase angle between the wheels is low, you will get little oscillation in the wheel. As one wheel travels further than the other, the phase angle will increase, allowing oscillation to occur.
This could happen with very little imbalance though, if the root of the instability is elsewhere, e.g. improper wheel/tire combos, looseness in steering components, etc.