There is the my .blend scene below:
http://www.pasteall.org/blend/35689
Just keep adding point lamps or spot lamps and notice the sun lamp get darker and darker.
Seems to be some interdependency with the clamping: You have set both “Clamp Direct” and “Clamp Indirect” to a value < 1, which you should imho never do. Reasonable settings for the clamping are either 0 (= off) or > 1 (the higher you can stay with the clamping, the better).
I did that and it did not work. I dunno what to do so I just created a new blend file and appended my assets and the problem is solved. I just need to do some minor tweaking now.
Funny, on my machine turning off clamping immediately solved the issue… Anyway, good that it seems to have fixed itself!