I never had the brightness fluctuations, but decided to update the logic board firmware anyway.
I have EU (Nordic, Finland) model PS59D7005.
Prior the update the Logic SW version was 11-02-11, main firmware is 1019.0, BT-G.
I had to use the ITOless_DeepBlack.bin, everything went smoothly during the update process.
Now the Logic SW Version is 11-11-04.
I haven't noticved any difference in image. As I said I hadn't noticed any fluctuations before, and the black level seems to be the same as before.
At least the update did no harm, and I'm glad I know now I have the latest version