It is usually the other way around, with Acrobat struggling to keep pace. It could be running out of buffer space. The Office PDF function is faster and doesn't use a 'printer' driver to create the PDFs.
I updated
Merge and Split earlier this week. See if the updated version of that add-in fixes the problem for you. This is a simpler process than
http://www.gmayor.com/individual_merge_letters.htm but no less effective at what it can achieve.