Test the water temperature, you have classic singes of the
nutrient temp being too high. The water/nutrient temp should be between 68 and 72 degrees. The attached picture is a picture of the roots of my strawberry plants last year, they were suffering from continues high nutrient temperatures. Wilting of the plants even though they have enough water is also another sign. Think of it like walking through the desert on a hot day and being able to drink all the water you want, only it's hot water. It wont bring down your core temp (refreshing), it will raise your core temp (heat exhaustion).
I can think of a few ways to cool the nutrient temp, some cost much money, some take constant attention, some take some work, and some money but don't take constant attention. The easiest and simplest is with ice, fill some 2 little bottles with water (almost full), let it freeze and stick the frozen bottles in the nutrient reservoir. This takes constant attention depending on how fast it heats up. Also insulating the reservoir from the outside heat can help keep the cool in (many different ways to do this). Also keep the reservoir out of the direct sunlight.