Getting FPS drops, a little confused why.

I sometimes get some lag when playing on maps that are not the test grid map. For instance I loaded into a map called Ghost-Town -http://www.beamng.com/threads/ghost-town.16405/-
AMD FX 8320 Eight core
8GB DDR3
And a MSI GTX 960
I have tried different cars but whenever i enter the city my fps goes down to around 13-20 fps
my monitor(TV) Resolution is 1360x&760