try rotating the tires, front left with right rear and front right with left rear, be sure they are inflated to the proper pressure and see if the problem persists.
If the problem goes away its either a tire imbalance, or a defect in the tire tread causing the tire to no longer be concentric.
If the poblem persists then it may be a loose tie rod end in the steering linkage, worn front axle bearings, or possibly air entrapped in the hydraulic steering actuator wich should be able to be fixed by filling the power steering resivor to its proper level, lift the front tires off the ground and with the engine running, transsion in park or neutral and parking brake set, turn the steering wheel lock to lock several times to give the steering actuator several full travel cycles.
