If your runoff is 7.9, that is way more than a little high. I'm surprised you don't have lockout problems but I'm glad you're getting lucky.
I am fighting a similiar issue with crap soil and using un-PHed tap water at 8.4 just a few times. I then PH-ed today's water down to 6.3-4 that was giving me a runoff of 6.7-8. I tested runoff, Ph-ed it down and ran it through again.
Did you add lime?
What is the PH of incoming water?
You may want to consider repotting.