Jiggle deformer problem on ball
 

So I have a nurbs sphere that I animated bouncing of a nurbs plane. It just does one single bounce and has squash and stretch on the bounce. I added a jiggle deformer to the ball. The effect was way to much so I turned everything down and it looks good with a slight jiggle.

Here's the problem: around the time when it bounces there appears a dark mark at the pole of the sphere. When I change the jiggle paramaters slightly it changes what frame it first appears on.

Anybody know what is causing this?

I've attatched a rendering.

Thanks.