Well try to see if this happens on other lots too. If so, it is probably a mod conflict or some new stuff you install your game does not like. You can check for mod/hack conflict using the hack detection tool found at the
http://www.simwardrobe.com site. To check, move all your downloads and load the lot, if the problem does not happen when you have no cc at all, it means something in your downloads is causing the problem.
If if happens only on that lot, it might have become corrupted somehow. You can either try the ffs debugger found at the MATY site
Here or you can simply move them out, place them in a new house and bulldoze the corrupted lot.
