It looks a lot like heat stress but from what your telling us it can't be heat stress. I left town for a month once and I had set up an auto watering for my soil mother plant. When I got back the auto watering thing I had set up had been working but, it had not given the plant enough water. Enough was given for the plant to stay alive, but it suffered damage that looked a lot like yours.

I would check the water lines to make sure that Sara is getting properly flooded and drained. Then move it 6 inches away from the light and put a small fan blowing directly on her. Then get some florakleen and flush like mad and give some fresh nutes. There could be a clump of hardened nutes in the hydroton, poisoning your plant. You need to dissolve it and flush it out.

Last thing i think it could be is damage to your main stem. Inspect it well, if you find something seal it up with some store bought stuff or superglue.

Good luck