What are your maximum and minimum temps running? I had an issue like that when I switched from cfl's to 1200 watts of HPS. My older plants were fine, but my little ones were showing heat stress. The symptom was similar to yours with a distinct yellowing near the leaf stem. I also showed the darker veins. Once I started to get the temperature differential smaller from lights on to lights out, they started to improve pretty quickly.

Don't take my advice as know-it-all advice. I've got all the basics down, but I'm learning the finer points of environmental control now. I don't know if you have temp issues or not. It's just that the last pic you posted looks somewhat similar to the way my stressed plants looked, so I thought I'd ask ya.