Very likely a change to your system done automatically (or manually). I’ve had weird automatic updates that like to interfere with my FPS (such as an old bug with the Logitech GHUB). Not necessarily your fault, but so many things are automatically updated it’s hard to keep track. Lately, I’ve maintained a steady 50-60FPS without any annoying issues. Usually a quick fix is to revert your Nvidia drivers and re-download them.