3D Rotation

I have been using quaternion mathematics to rotate a 3D model. Initially it works fine but after the model has been rotated, it seems to develop a jitter bug. Has anyone encountered this problem and if so could you tell me what the solution is?

[Gavin]

Are you normalising your quaternians properly? Floating point errors? do you do anywhere...

float x;

if (x != 0)
{
.....
}