(The plants sit in small four inch pots with aquarium grade stones.)

This is the only thing I would question. You sound like your doing your homework well. And trying many times with same results. Those stones sit too close toghether and do not hold water for a cation exchange. The roots by flower time may be like root bound and choking off by then. That's why they do good in veg, plenty of room.

I would go with another medium. Hydrotron or rockwool or lava rocks just to see. You have studied and tried almost everything else.

Do a search here for Loc Nar. I started with drip system with 6 plants to a 5 gallon tub. Only when I change nutes did I have to down my PH for a day or so.

Pic's of plants and roots would help greatly. You have come too far to give up now. If you over come this it's bud city. And what's the lights and heat in your room and rez ?
LOC NAR on probation Reviewed by LOC NAR on probation on . Low PH problem First time poster, Long time lingerer :p These problems have been occurring for a long time and I have never found a solution so the plants have suffered for some years. :( Right now there are some plants growing in a homemade tube system almost exactly like pictured below. The ingredients consist of PVC fence posts, PVC pipes for the manifold, plastic reservoir, PVC glue to hold the manifold together, Plumbing grade rubber tubing to connect the Rubbermaid top-off reservoir, a pump, Rating: 5