Well, there is a reason people like ebb and flow and/or flood and drain - And there is a reason people like me will never touch it, haha....

I have had issues in the past where my RDWC would produce a browning, and typically it was when my water res was too low and too much air was getting to the roots. Whats your flood and drain timer at? Is the timer different in veg than it is in bloom?

I'll help you find your problem