I would also stop the use of distilled water. It's taking more buffers out of the water, and makes it easier for your ph to crash. But you want to do this gradually. What I would do, which is similar to acclimation, only do 10-15% PWC's for the next 4-5 days using just tap water with conditioner added (Dechlorinator). I prefer using Prime myself. Depending on the softness of your water, the distilled could be causing some of your problems, causing ph swings that you are not seeing because you use test strips, which are usually inaccurate. Get a liquid test kit. Same with ammonia, nitrite, and nitrate. Those are the 4 kits you should always have on hand.