First of all, get Service Pack 2, if for no other reason than to patch a treasure trove of wonderful secuirty flaws. Of course, you'll still need to get all of the 38-and-counting critical updates since SP2's release.
Second, make sure you have the latest drivers from ATI and the latest version of DirectX from Microsoft (currently 9.0c). The reason for this is to eliminate the possibility of bugs in the older drivers.
Third, as touched on by Volkier, you may have a heat issue. Make sure your CPU and video card fans are not clogged with dust and that they are spinning properly.
Since it seems to be only on certain parties, have you recently installed/removed a mod? Is it vanilla or a mod?
I still sometimes get crashes during gameplay, but most I can write off as the expected problems of using a beta: there's bugs not yet discovered. But, I have managed to eliminate a good deal of the crashes by lowering the battle size limit. Try this as well.