This is weird.
I've rendered out a composition to a TIFF sequence using media encoder CS5, at 30fps non-drop frame in settings everywhere, and it has inserted an extra frame every six frames. So 42 seconds becvomes 51!
I can fix this easily by deleting every sixth tiff, but this would've been a pain if it had been a video sequence.
What's causing it?
Have you accidentally enabled the inverse pulldown associated with some digital cameras that record in fake 24p? Sounds very much like that. Otehr than that it could of course still be soem sort of bug where the routines mess up the framrates....