Are you using a widescreen hack?
It is simple to understand. When you move your character around, the programmer must show nearby objects for you to see and remove far away objects to free up memory and reduce GPU load.
The game itself was programmed to output 4:3 aspect ratio. The 16:9 patch modifies the game, forcing it to show objects on the left and right sides of the screen, which is not meant to be shown.
There was a post on the widescreen thread 'bout the solution for this, but I have not tried it. I thought it is a bit complicated (or maybe I'm lazy).
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