Hello simon,
You have me stumped also. They sure seem to show signs of heat stress, especial when they dramatically wilt in the sun. But with an air temp of 75-80, water temp of 65, and the water level being good as well, it obviously isn't due to heat (even in full sun). My second thought would be a disease and/or fungus, however not if it's only happening when the sun hits it.
Turgor pressure is something new to me. I do know that a loss of water pressure within the plant tissue will cause wilting, but never knew the specific term, nor do I know everything that will cause it (I have only dealt with heat issues so far). Reading from the link I posted it also suggests that wilting can being caused by salinity. Are you using tap water? Also it suggests wilting can be caused by ethylene (a plant hormone), but I'm not sure if that is related to the salinity of the water or not. I will try to look into causes of the loss of Turgor pressure.
P.S. I will send a e-mail to a tech at General Hydroponics with your situation, and see what he thinks. He usually gets back to me within 1 day during the week.