In game go to the main menu and go into options then on graphics and set the level of special effects to low, shadow effects to none, water effects to no and fog to no. Apply these changes and you should no longer have a problem.
If the problem persists go back to the menu and in the graphics section of the menu click on advanced. In this menu put filtering on bilinear or none, sync on deactivated and texture quality on low. Apply the changes and go back to the game, you should change these options until you find one that is stable for your system.
If you have lag like problems (where the game is skipping) you may want to lower the resolution in the graphics menu to 640x480 then return to the game then raise the resolution until you find one that gives you the right mix of looks and performance.
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