I've had similar problems although not to that extreme. What seems to work for me is to flush the plant really well, I mean way more than you would normally. I put mine in the tub and dump about 20 gallons of water through a 3 gallon pot over a 1 hour period, I really make sure it's totally void of nutrients. I then give it a full feeding of a complete nutrient like GH or Dynagro and wait for the results. I would probably do a much lighter feeding, maybe 1/2 strength in your situation because of the age of the plant. It takes about a week for things to turn around and start improving. I just went through this with a Sensi Star cutting and she has really sprung back and is healthy again. The problem was a nutrient imbalance, which was causing a lockout, it couldn't uptake several of the nutrients.
Cut off the dead stuff. I will say this may not work for you, I don't use soil, I grow in a soilless medium.