Conquests low frame rate

[Griffinite]

I bought the complete edition of the game a while ago and everything worked fine, except that playing a game in conquests has a big drop in frame rate compared to vanilla and PTW. Only just started on Civ4 with both expansions and BtS runs fine and smooth, even though it should be the game that needs the better hardware. Anyone know the problem and a fix?
If it matters, the drive civ3 is installed on is almost full and the Civ4 install drive still has free space.