Does the juddering happen when turning both left and right, or only in one of them? The reason I'm asking is that some years ago my VX Cavalier started juddering and jumping to the left on right hand bends and corners, really scary. Previously I had kerbed it hard on the nearside to avoid a head on collision with some nutter cutting across a bend. The car had to go back to VX to re-set the steering/suspension and tracking alignment. As you have only had the car a month, it is possible that the previous owner may have kerbed it or something similar. If you haven't already done so, I would have Kwikfit (fairly cheap) or another fitter check and adjust the tracking if needed. You also have a fourth tyre different to the other three. It's usually recommended to have two matching tyres on each axle, so it may be better to put the odd tyre on the rear, or replace it. Hope that's of some help.