Do you have fixed bounds set on your particle system? If your bounds are not set correctly you may get odd rendering issues, especially with GPU particles when you reach the edge of the view frustum. If you exported into a new effect I would expect the bounds properties to transfer over, but perhaps there is a bug? Please make sure your GPU effect is within the bounding box of the effect (click the bounds button in Cascade.)
Is this only happening in the Cascade viewport?
Does this exhibit the same behavior outside of Cascade when placed into an empty level?