It never happens with XSI 7.01, with Softimage 2010 it happens sometimes, with more than 10000 points per cloud. With SI 2011, it seems it appears all the time, when count of points is more than around 2200.
Because I didn't played with SI 2011 yet, and I got this info from another people, here is a simple setup done with factory nodes only, that can cause this issue. Create a grid, go to Simulate>ICE>Emit Particles From Selection, delete simulation region in stack, set rate to about 3000 per frame, set Particle shape to segment, set Rasterizer for primary rays.
Cheers
