I had very similar symptoms, lower leaves yellowing, very tips of leaves dying, leaves curling down like a claw.......As best I could figure it was several factors that were off.....Too much heat, low nitrogen (possably to nute lock out) and low PH...the one that threw me the most was PH.....I flushed ( I put two small fans in after flush to help soil dry out as I was worried about adding another problem of over watering), tested soil PH and watered (after soil dried) with a light nute solution and adjusted the PH (aquarium ph adjusters added to water).....As for the heat i went all out and built a new cabinet, but adding some extra ventilation should do, if you don't have a thermometer, get one.........This all happened about 2 weeks into flowering, at 5 weeks now and things are looking better, yellowed leaves did not get green, but new growth was good and things did not get worse......Good luck with it.