trollmage --
I regularly get a yellowing and a die off of the large main fan leaves that hang off the center trunk late into flower. You have to remember that flowering is a very stressful time in the life of the plant. Nutrients are being turned from growing leaves, stems, etc and being diverted into the sexual reproduction of the plant. Just think about how desperate you get sometimes for the soft sweet caress of a female. Well, the plant is yearning too, but for some male pollen, and its just growing lots and lots of those juicy little flowers. I've tried increase nutrient concentration and feeding twice a week (I tend to feed about every three or four days and use one fluid ounce of Age Old per gallon) to no avail. Also, I use air conditioners to hit the plants with cool air during the dark cycle and I think that tends to accelerate the browning of those main fan leaves.