Also, yellowing leaves in week 5 of flowering are not all that uncommon. Part of the plants life cycle.

Zompton has good advice. Also, I've had some luck at the stage you describe with doing a light nutrient flush (400 ppm) at 6.2 - 6.5 ph, then back to your normal schedule when the plant needs watering again. How hot are you mixing your nutrients (ppm) and what ph?

Peace, Farmer Rich