Do you have chloramine in your tap water? Here's something that was happening to me that sounds like it could similar to your situation.
Chloramine is a mixture of chlorine and ammonia. Even after using a dechlorination product you will still be adding ammonia to your tank. (In my situation, it was quite a large amount of it)
Your nitrifying bacteria will "consume" your KH as they convert the ammonia in the water to nitrIte then to nitrAte. When your KH gets to 0 you have a very low and unstable pH, hence the term "pH Crash"
In this situation, as you have seen, water changes do not really help, because although you add KH by doing the water change, you are also adding ammonia by means of the water change which is going to deplete your KH again anyway.
The solution as Rex mentioned would be to supplement your tank with additional Carbonates to raise the KH enough to negate this effect. Baking soda is the simplest option.
Keep in mind that I am no expert, and that I cannot guarantee what happened in my situation is what's happening with yours. Based on what information you've provided, I'd almost be willing to bet on it though. Anyhow, if this turns out to be the case, you might want to check your nitrAte levels, as they could be through the roof.