Inconsistent font display in PDF from InDesign (using SAME font)

There is a noticeable difference from one page to the next in some of the PDFs I'm generating from InDesign (exmpl: Light black font against white background). The curious thing is - all pages are set-up in the InDesign document exactly the same (roughly 4-8 pages in each file - text simply flows from one page to the next), it uses the same fonts. However, when I go to write the PDF (I've tried several different Acrobat PDF output settings) - the resulting PDF is not consistent. As I scroll through my PDF file - I notice that one or two pages look outlined!? Completely random, and I've been using these same files/templates (InDesign) for YEARS and this JUST started happening. It is critical for me to be able to produce consistent PDFs since my clients view their files on-screen as-opposed to printing them out. Anyone encounter this issue? Any ideas what might be going on here? My fonts aren't corrupt (reloaded them anyways), my InDesign file is clean (don't see anything obvious there - I never changed any settings for this to start occurring < very simple 8.5 x 11 basic cascading multi-page file), I'm at a loss... PS - I also tried generating PDFs from a CS5.5 environment and then from an updated CS6 set-up on a completely different computer and the results are identical. Out of 6 pages - one obviously looks like the fonts have been outlined on the screen (in Acrobat). I'm baffled.