Civ IV suddenly lags

[Ozymandias69]

Hey, folks. This problem confuse me as the game(both Vanilla and BTS)used to work perfectly fine even using the best graphic settings. I remember only several weeks ago i was still able to play on the huge map with much AI and with my air units all around along with my naval units surrounding the planet on the later part of the game(a bit slower though, but still very playable). Suddenly, several days ago, i lost that epic moment. The game became very laggy during the industrial era in standard map and only 6 AI. There's no major changes in my PC either. I did not do any software or hardware change. No new programs installed. The lag is also kind of weird because it doesn't lag during combat. It doesn't slow down the turns much either. Even when i put up the diplomacy screen, the movement of the animated leaders works fine. The lag is only in the movement of the units.

Oh, i almost forgot. I just remember something. I did once keep uninstalling and reinstalling the game for maybe more than 10 times because for unknown reasons i could not play BTS and when i try to reinstall it it couldn't find the Vanilla so i reinstall the whole thing until it works.

So, what do you think, folks? What should i do?