Got a picture?

For soil grows, (Sunshine 4 is in the soil category) you'll want the ingoing water to stay between 6.3 to 6.8 ph. Shoot for the 6.8 mark for a couple of waterings, and check the runoff. Speaking of which, you aren't using aquarium ph drops to check your runoff, are you? The color-coded result is skewed by the tinting in the water making the test result inaccurate.

I adjust my 8.0 wellwater to 6.8 with phDown. (phosphoric acid)

Soil buffers (lime, limestone dust, gypsum) that are added to commercial mixes fades over time. Usually lasts about 2-3 months, and your ph will start to crash. Doubtful it's your runoff ph taking a dive, more like an inaccurate testing method.
Rusty Trichome Reviewed by Rusty Trichome on . ph run-off @ 5.0 What is your experience level? novice Your Equipment: .1) Type and wattage of lights? Dual core horti 1000w HPS .2) Distance from tops? 30? .3) Reflector type? Super sun 2 .4) Is there a consistent fresh air supply? Yes .5) Do you have an exhaust fan and a circulation fan? 440 cfm ecoplus supreme inline and 2 18? oscillating fans .6) What are the bulb wattages, Kelvin ratings, and schedule? 1000w HPS 12/12 Your medium: Rating: 5