This is kind of a dead thread but just in case people are still having this problem, Even using Autocads DWG to PDF.pc3, (like us) I found this
http://forums.autodesk.com/t5/AutoCA...F/td-p/3508916 link that has what appears to be an autodesk employee telling us that it is a known bug. Since its a bug, I got to thinking, DWG to PDF.pc3 treats solids as vector polygons, but treats gradients as rasters (I.E. ole objects, or x-refed images) I converted a solid hatch to a single color gradient of the same color and voila, no striations. as a bonus for those not using DWG to PDF.pc3, since the hatch is a raster instead of a vector there wont be any triangulation lines.