Shaking driving wheel when moving

[Eroze]

Hi, got a little problem . from time to time the driving wheel starts shaking, quickly vibrating, but with no obvious cause. Same movement like when the tires are bad or there is a "roller curtain" on the road. It apparears and disapears just like that,speed doesnt not influenc that... does that sound familiar to anybody? should I stop using the car immediately?
thanks a lot!

[pommy]

Jack the front end, grab the top and bottom of one of the wheels and wobble it from top to bottom, not left and right

...Could be a ball-joint.