"Alligator skin" on some horizontal surfaces.

I have a Replicator 5th generation. On a few recent prints, I am getting an alligator skin effect on large horizontal top surfaces. Everything else prints fine including smaller horizontal surfaces.
Print settings are 0.3mm, 4% fill, 2 shells. I realize these are very light settings, but the parts are for a UAV and we have been getting very good results otherwise.

Any suggestions would be most helpful. Thank in advance.

Attachment 6907