The dotted lines are stitching artifacts resulting from transparency flattening—PDFs that are exported to Acrobat 4 Compatibility (i.e., PDF/X-1a). Stitching lines are only visible with low resolution output, or screen display.
When you view in Acrobat, uncheck Smooth line art in Prefs>Page Display>Rendering to hide the lines on screen. They will not print when the output is high resolution.
If the PDFs are only for screen viewing, try setting your Export>Output tab to this:
