At week six of flower i'd be upping my numbers as well. 700-800 isn't anywhere near the 1350 my plants would be at that stage. (in fact....my plants would have been on more than 800ppm since about the fourth week of veg.....) I'm pretty sure that's where your low yield is coming from.

As for the ph dropping.....I'd be concerned for root rot etc as X-C says. You are late in your grow so it may be normal here to lose some roots (which can drop ph) but that's a fairly good drop.