file corruptions.
both the baldur's gate series and iwd series are famous for this.
first off are you patched?
secondly, sometimes the corruptions can be fixed by emptying out the temp files. so try emptying that out.
but i would venture to say that your corruption has occurred in your savegame. and there's no cure for that.
except to start over or go to an earlier save.
good luck!
V***V