Illustrator :: Lines Being Printed That Don't Appear On PDF Document

I own a company that designs and sells sewing patterns for men's clothing to be made by home sewers. I have my patterns professionally drafted and sent to me as a poster-sized PDF. I then open it in Illustrator, move things around, change some fonts, etc... and make a 34" wide copy to be printed at a copy shop. That's all fine and dandy, and it prints out perfectly.
 
I then use Acrobat XI Pro to open the 34" version and print to Adobe PDF with the poster setting of 7.5" x 10" for people to print at home. Then I open each page of the multi-page PDF in Illustrator to add a border, alignment marks, page numbers, etc... and save as the same PDF.
 
Now here is where the weirdness begins. For some reason, there are random chunks of lines (identical to the pattern lines throughout the rest of the document) that aren't supposed to be there. They don't show up on my screen when I open the file in Illustrator or Adobe, but they get printed every time. I've even started the whole process from scratch, tested the 34" wide version at the print shop (prints perfectly), and split it into pages again, and the phantom lines are still there!
 
I've attached a photo I took of one of the pages in question. The file on my screen is the exact one I printed, and as you can see, they are definitely not the same
 
I have been saving the PDF files as Press Quality from Illustrator.