Very slow game speed Options

[Shadan]

I am planning to play an another BG2 run while we are wating for v6 release, but I have one serious problem. In all of my BG2 games (was 3 of them with IA) I experienced a very slow game speed in certain situations.
1. When there were lot of enemies and/or corpses on the screen (like githayanki suprem leader fight).
2. When there were many spellanimation on the screen (like SI, Mirror Image, GoI, Spell Turning etc.).
3. When I fought against big monsters (like any dragon).

It was very frustrating sometimes. First I thought it is because I have some mods (like bottomless Bag of Holding), but later I had only IA and The Four installed after ToB patch, Baldurash and EoU, and I dind't have bottomless Bag of Holding at this time. Despite this clean and modfree IA install, I had very slow battles still. My computer is not the newest, but I have 2 giga memory and I can play all games, even WoW without any difficulty. Can anyone give some advices how can I avoid this slow speed in BG2?
Ah, I played in 1024*768 resolution, but I tried in 800*600 also to check if this could be a problem.

[Shadan]

I am at home now, I have NVIDIA GeForce 6600 GT video card.