Suspension/Shock Problems?

Hey guys,

Just had my steering rack replaced on thursday which is probably unrelated to my current concern but noting it none the less.

While driving on the N3 today and going over some of the "joins" in the roads at around 130km/h it felt like the car was "twisting" at the back. The feeling was very similar to being passed by a big truck on a windy day where you need to hold the steering tighter as it sort of waves around. Any ideas?

Thanks in advance