In my experience, strips are good for ball-parking pH, but they do go bad after a while. I am in the South, so humidity is a big issue with them. I have used meters, and still mainly do. Regardless of using cheap china junk to expensive lab grade meters; they all need constant calibrating. I'd recommend a middle of the road meter and a pack of calibrating packets.
Edit: Sounds like you are getting a pH crash with too fast of a fermentation. There are buffering agents you can condition your water with to help stop this. They can affect taste, though, so it does take some experimentation.