You can bet it's your driftwood, especially if you have a low buffering capacity in your water. I have the same problem, luckily for me my tank is a south american tank so the fish love it. The type of wood that I have is malasian drift wood and it can change your ph relatively quick, depending partially on the size of your tank. I would still agree with mistergreen above though as far as the water changes. Sometimes substrates can effect your ph too. Run some plastic cup tests over a 24 hour period with some of your substate if your using something other than gravel. Also try it with your regular tap water because your initial tap water ph can be a fake or temporary one. Mine will change a decent amount over a 24 hour period all on it's own. Good Luck!