When I enter cities get heavy FPS drops every now and then. It drops from a steady 45-60 FPS to 30-45 FPS. By alt-tabbing I get it back to normal.

1) Is there any way to prevent these FPS drops when entering cities?
2) Is it normal that alt-tabbing fixes the FPS like this?

My hardware:

i7 3.4GHz
GTX 560 Ti
8 GB RAM