Dark Brown/Black Terrain makes game SLOW (Pics)

[Kairu]

During the game, after a while, the terrain will turn a darkish color, that looks like this

This is a problem because when this happens the game goes so slow, and I can't even play it. I am playing on a large map, I have a Geforce 6800, 1gig of ram, 3 GHz processor. I don't think it's my system. Is there any option I should turn on/off, or does anyone have a solution to this problem? It's a pain in the ass. It goes away if I save and reload, but after 5 or 10 minutes it turns the dark color again.

 

[Zanmato]

Could it be your card overheating perhaps? If it corrects itself when you close Civ4 then does it again after several minutes when you reload my money is on a heat problem.