check the temps on your gpu. it's very likely to be overheating, and that is what is causing the frame drops.
use something like "open hardware monitor" (just google for download spots) and run it while you are playing the game. it will tell you where your bottleneck is (gpu or cpu).
also suggest trying out a frame limiter for your graphics card.
for nvidia, there is "nividia inspector". there's one for ati cards too, but don't know the name.
since you're playing on a laptop, limiting your framerate manually may actually make the game smoother.
again, use hardware monitor to see where the bottleneck is. if your cpu or gpu are getting overutilized, you know where to focus your attention.
