The "what you see is NOT what you get" bug. Happens a lot in Word.
When you create a document in Word, it always adjust the document slightly on the screen to match you default printer. If you create the document with one printer as default, and then switch to another, it can cause printing problems. Same if you switch printers back and forth while creating a document. It will do funny things with the document.
Recommend: Go through and put in a section break about every fifth page. Then if something happens that alters the fonts, margins, etc, that is the furthest it can go.
Then, print the sections one by one. When you find a "problem" spot, then select the ENTIRE section (all five pages) and reset the font, margin, etc. for the entire section. I know the feeling - you can't see what is wrong, but the computer can. Often when we try to fix it, we don't select the spot where the problem really is becasue it looks alright to us. So always select a bigger area and reset it.
Finally, if all else fails, create a new document with the font, margins, etc, you want set up. Open the original document and resave it as a text file (.txt) Note: This will NOT change the original Word document, it will just make a new file in text format. Open the text file, and copy the text about five pages at a time into the new document. This will keep the text, but remove all the formatting. Finally, go back through and reformat the new document the way you need it.
Good luck....