Shuddering (assuming you mean what I mean by shuddering) can be caused by a number of things.
1. Too much caster (alignment). This is like a shopping cart with one of those wonky wheels.
2. Warped brake rotors.
3. Warped wheel.
4. Steering gear or motor problems.
5. Something else (road surface, roof rack, drive train, etc.)
1) So it doesn't feel like an alignment issue. I've had several cars with that problem and my car was having that problem over the summer. It feels different.
2) I just had the car in for Service last week, so I doubt this would be the problem. Plus, it doesn't happen when I'm using the break, just accelerating.
3) I'm not sure on this, but like I said, the car was just serviced. Also, it hasn't just happened since being serviced so I doubt it's anything that may have happened while it was being service.
4) I'm afraid it'd be this.
5) It's definitely not the road surface. I can go over the same patches of road and not have the problem, which I've done many times. Plus, when it was really dramatic I was on super smooth road, and I've gone over that a bunch more to try to get it to happen again and driving the same way and I didn't feel anything.
Try to notice what you are doing when it happens. Is it over a particular patch of road? Are you braking, turning, etc.?
I thought I had it down to certain situations then on Sunday it through me a curveball where it happened really dramatically and didn't really match any of the situations except for low speed. The "vibration" feels more in the direction of travel, like rapid changes in acceleration, and also very rythmic. At the same time, the little whine you hear at lower power settings starts making an oscillating sound that seems to match the shuddering, though I've noticed the same sound before without the shuddering. But really, from past experiences with the car, I have a feeling it's going to be something pretty major, though I'm crossing my fingers that it won't.
Also check the torque on the wheel nuts. They should be 175 Nm / 129 ft.lb. Too loose and they can do this.
I'm going to have the car in for service again on Friday to have the security module replaced, so I'm sure they'll check the torque on the wheel nuts and such, since they know about this problem already. I'll make sure they do, though.