Lagging when moving units

I've played a lot of civilizations 3, but I recently had my game screw up when I opened it up after a few months of inactivity. I have the complete edition on Steam. I run Steam and all it's games on an external drive connected to my laptop. It's always ran fine, until I started playing again a few days ago. The game runs fine on my Windows 7 64bit, but whenever I hold down the button on a unit and move it, to aim him to go to a particular hexagon, the game immediately slows down to a weak 2 or 3 fps. It stops after I aim him, but it's frustrating how impossible it is to move units now. Anyone have experience with this bug, or any suggestions?