Help....do you have an oscillating fan running and are you using hydroton? I've been fighting the exact same issues in my last few grows and can NOT figure out the problem. My ph is normal too....I check it all the damn time. Do you have any photos?

Edit: I'm using the botanicare nutes and my ppms are up around 1400 so I know its not a lack of available nutes.....although your numbers are lower than mine.