Does it happen in subsequent battles (ie: if you CTD, reload, fight that battle, then go back to the campaign and start another battle, will it CTD again), or is it only the first battle that will do it?

Could it be something to do with the campaign rather than the battle? Im assuming somehow the campaign gets sort of turned down during battles to free up memory etc, and perhaps thats not happening properly.

I dont really know how these things work, just trying to offer other possibilities. The fact that it works fine when you reload seems to rule out unit/trait/general problems, or problems with the battle system itself.