Allow me to offer another angle towards looking at the problem.
I had very similar problem to what Janusushi described. I was at the stage when party first entered baldurs gate (chapter 6
BG1 of
BGT).
I found that the lagging can be alleviated gradiently when my "container items", such as bags of holdings, ammo belts, potion cases etc, are gradully removed from party inventory. Eg. I had 8 containers to begin with - jerks hard always. When I remove down to 4, jerking is minimal but can be felt. When down to only 1, almost no jerking.
The changes in smoothness of the game is astoundingly quick...all it takes is return to main game view, and move about your character and pointer, and straight away you feel a difference.
So yah there you go, if you have a similar scenario to mine, try trashing those containers...