Pooling effect, you mean the HOM (Hall Of Mirrors) effect? Wich means the background isn't redrawn so the last thing that was drawn there stays visible (a bit of a flickering effect).
Anyway, what you need to do is change the Max viewable distance. In the Hammer menu, go to Map, then Map Properties, then click on Max viewable distance and change the 4096 (the default value) to something higher. Something much higher when your map allows the player to see from one end to the other. It's measured in grid units bytheway.