Your driftwood is pulling your pH down and you have little buffering capacity.Maybe a coincidence but I have 3 tanks with Malaysian driftwood and 3 tanks with Mopani driftwood. Two Malaysian tanks are bare bottom, one has eco complete. Two of the tanks with Mopani are bare bottom and one has eco complete. All three tanks with the Mopani have stable and relatively high 7.6 ph, the ones with the Malaysian consistently drift down to a yellow reading on the test kit.
A recommended way is to add a bag of crushed coral to your filter..
On a side note.. That is a fairly big drop even for driftwood..
Personally I'd pull it and soak for a year or so.. half kidding..