I think I figured something out.  I had the cloud scaled about 4200 in X and about 4800 in Y with only 100 in Z.  That seemed to work OK in C5 but not in C6.  If I bump up the Z scaling, the grain seems to go away.  It seems that X and Y can't be scaled much above a 2:1 ratio campared to Z.

As you can see in my screenshot, this is the same C5 file open in C6 with the only change being the Z scale parameter.  I'm happier now that I have figured it out.  Just something to be aware of.