Spiders problem

[DRedshot]

I remember that an explanation was given about raising some sorts of collision box in the model editor. Of course, this will mean that you will have to create a new version of the spider, or else your mod won't work on other peoples computers.

This is true. The spiders collision box clips with the floor so it cannot move. I can't find the video which originally demonstrated it though. Just open the spider in the model editor, raise the collision body by just a fraction, so it isn't under the floor, and save as a new spider.
Then stick it in your custom story folder (I think it's myStory/entities/critters/spider/ but don't quote me on that! ) and people can use your new spider!