PDFs sometimes print with strange symbols

My organization's mailroom often has to print hundreds of documents at once to mail out. Every 40th to 50th document printed comes out with some letter changed to random symbols. Reprinting such documents on their own works fine. The printer drivers as well as Acrobat Pro are fully updated. It happens regardless of what printer they use, Ricoh, Brother, and Cannon. It doesn't seem to be a font issue, as the reprints would end up with the same symbols if that was the case. In smaller volumes this might not be a big deal, but having to go through 200 documents to check for the possibly 2-4 that didn't print correctly is a huge hassle and reprinting causes addresses to be out of order. I'm hoping someone here has some ideas about this.