Has the driftwood stained your water at all? If you put some in a glass and hold it to a white background, do you notice it's off color? It's quite likely the driftwood causing the pH drop. It will lessen over time, but what you can do is boil it for several hours if it will fit in a large pot. This will cause it to release a lot of the tannins, meaning less going into your aquarium!
Also, I believe Fluval Stratum has buffering capabilities. It is going to buffer out your carbonate hardness, resulting in the pH dropping further because of the tannins.