Actually a bit of fluctuation is something good; different nutrients are available at different pH levels so having your pH fluctuate "a bit" makes more nutrients available all together hence a more healthy and nutrient rich plant. Of course this is only a bit of fluctuations, not from 4 to 7 in one day.
About the droopy leaves; usually a sign of over-watering although with a Waterfarm the only way that would happen is if you turned your pumps off. The WF pumps must always be running 24/7. Another thing could be that your looking at them right before they go to their night cycle. Plants tend to get accustomed to their schedule and after a while they know when the lights will be going off and sort of calm down and drop their leaves just how someone get's tired after a certain time of night they start to get drowsy and sleepy. The opposite happens when they are just about to enter their light cycle; it's leaves shoot up into the air and spread getting ready to receive the light.
Anyway hope it helps friend. Keep us posted.