Steering wheel vibration
 

I just got a 2009 ford escape v4. The problem i'm experiencing is that when I drive it, unexpectedly, the steering wheel shakes violently for about 1 to 2 seconds. As soon as I felt this i took it to the dealer but they weren't able to "reproduce" it. the shaking has increased and still can't find the reason. does anyone know anything about this?
Thanks a lot.