I would suggest you download something like MSI Afterburner or a similar programme to monitor specific information when gaming. In and of itself, I can't think of a reason why you're suddenly getting lower fps; the obvious reason tends to be CPU temperature being too high and it throttles.
Whatever monitor you're using I would likely choose things like the GPU usage, GPU temperature, CPU usage, CPU temperature, RAM usage, fps, frame times to see if there are any correlating incidents appearing.
Given the graphics card you have, I would suspect your CPU usage to be quite high.