When you use OHII full or with new ships - there is an extra layer added to the game that adds traffic to the ports. So if this is happening within the 30km radius this is the most likely cause.
You pc specs should handle OHII full with no problems
Check out you shadow settings in the graphic options in main menu and try to turn down a wee bit. Also try vsync on or off.
Antilag mod may also help if you dont use it already
