Sounds to me like a heat/humidity related prob, when it's to hot/dry the plants transpire to much moisture to the air, the leaves curling up at the tips and margins is usually a last ditch attempt by the plant to retain what little moisture it has left in the leaves, what temp. and humidity do you have? Maybe a humidifier will help, marijuana is very temperature tollerant and as long as you have a good humidity level they should be able to withstand quite high temp. Hope this helps, good luck mate.