Something is causing your roots to die off. That's what's lowering your ph. Root death causes acidic conditions......overnute or burn generally raises ph but then it will lower again as the dead root disinitegrate (sp?) If you are not using a good root product for root health I'd start asap.

Stinky is right that you must correct the ph abnormality first, then figure out why it's happening. Bloom nutes ARE lower in ph generally but your job is to keep things balanced. Escpecially as your plants get older (and yours obviously look pretty darn happy) you WILL have to do more hopping and jumping to keep things correct. This is a normal part of the plant's life cycle......some root death is normal here as the plant completes it's life cycle.

PS....SILENCE!! I KILL YOU!!!!! I love achmed.

Do ou