pH issue and general confusion (sort of long)

[Flarkington]

this is my 5th grow, so i'm not absolutely new to this but i'm having a problem i've never had before.

i'm currently in the end of the 6th week and the pH in my res. is fluctuating at least 2 points every day. every morning when my system turns on i check it and its almost 7 so i lower it to about 5, and the next day its back up again. until this point in the grow i have been able to set the pH and it didn't change for at least a week.

i'm worried now because i didn't catch on to the fact that this was happening until about a week ago, so my plant has been consuming its leaves for longer than 2 weeks. i've removed a lot of yellow leaves, and there are a lot more that are yellowing.

my biggest worry is that the plant is focusing on fixing the yellow leaves and not making buds, so i'm wondering if i should remove all the leaves that are starting to turn yellow too.

i really am stumped at this point, beyond just changing the pH everyday.

Any thoughts?