If the ski does this when on the flats, then it's normal and can be alleviated with slight edge pressure. Almost all modern skis will do this, and it gets worse as the sidecut gets deeper. The wider the tip gets relative to the waist, the more the ski can yaw and wobble around when perturbed by irregularities in the snow.
If the ski does this when edged, I can think of two common causes. Most likely -- you are not keeping your weight forward and over the edges. For instance, if turning across a steep trail, the tip of the downhill ski can wobble if your body weight is not projected down the fall line (say if you're leaning into the hill). Sometimes tip wobble is my reminder to keep my upper body leaning down the hill.
Second, I have noticed that some skis have jittery tips when I first ski them new (or after a tune), and this is always remedied by lightly dulling the tips and tails. It's not always needed, which is why I wait until I ski them for a few runs. If I feel like the tips shimmy around and are grabby when edged, then I dull the tips. If not, I leave them sharp.