experience will indeed stop accumulating if you're too high of a level above the enemies.
suggestion for maximizing experience if doing the BFs:
in each act, after you find the first BF key, depart for the BFs. kill every monster above ground and in dungeon 1. from that point on, immediately (well, at least soon) upon finding a subsequent key, do that dungeon.
by proceeding this way i experienced minimal 'killing without experience'.
if you find yourself not gaining, leave. there is truly nothing of value in the dungeons except experience, and continue with the main story. try to proceed to the next key WITHOUT gaining level.
this becomes especially true in act IV...
[color:"yellow"] SPOILER ALERT!!!!!!!!!!!!!!!!!!!!!!! (proceed with caution) [/color]
...where there is minimal killing taking place and the experience is derived from puzzle solving - this means that you should gain experience by killing all the BF stuff first (which IS dependant upon level) and leave the 'set experience' puzzles for last (you'll get this amount regardless of level).
