I played this game on my Radeon HD 5770 and had no problems. Framerate was always smooth. When I upgraded to a Geforce GTX760 the problem described here in this topic began. Even on the settings I previously used for my HD5770 I still get framerate drops that just didn't happen before I upgraded to the GTX760, and those drops happen exactly as the first post describes. Just wanted to add this information because it might be relevant. Could it be a problem for nvidia cards only?

PS.: The only thing I upgraded was my video card. Everything else remains the same.