It seems to me that if your starting pH is 7.8 and you are still experiencing pH drop, there are some other issues at work. My well water is 6.4 and when mixed with food it drops to 5.8 which is what I feed with in coir and never have to add any pH adjusting chems. In my NFT systems, I experience a slight upward shift when the plants are large and need to add a bit of acid to the res.
If you were to do a really heavy flush with straight tap water, you should be able to raise the pH substantially. As soon as it looks normal again, I would go back to a regular feed to bring the runoff back to the correct ppm's and then check everything 24 hours later and see where it's at. If there is a huge drop in that time then there is definitely something strange going on in your root zone.