Is your game up to date with the patches? There were several versions of each patch released. I'd recommend saving quite often while building, every change you make has to be remembered so it can be 'undone' if necessary, saving will probably help clear that temporary info, another method I use is to flick to the options screen and back again once in a while since that makes it forget too (no idea what I'm talking about actually - this is guesswork), it seems to work for me at least. There was a problem while decorating the Stage (the foundation type thing that came with Open For Business), it used to crash sometimes by just hovering over it with the wallpaper tool (usually when railings had already been added), the patch should put a stop to that.
Otherwise just Save often
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