If there is a 'debug' mode you can enable on a sim that helps you figure out what the heck is going on here and thus fix it faster, let me know, I'd probably volenteer my sim to have this debug mode enabled to help you track down the source of this problem.
I have a theory that it happens when lots of avatars teleport in/out at the same time, but that's just a theory. I've been told the coders are really trying but ultimately don't know why it happens.
I'm very, very motivated to get this bug fixed as it is causing daily headaches for my sim and everyone in it (e.g. one parcel with 16k traffic, where just yesterday we had to restart the sim just before a major event was due to start). So, if I can help do ANYTHING to get this bug fixed faster, let me know.
I already file bug reports when it happens, and when the sim (as it usually does) crashes a short time after the bug appears to go away. I encourage everyone else to do the same. Last time it happened I invited a Liaison over to observe it. It disappeared when she got here. A few minitues after she left, the sim crashed. Maybe that's the solution? Have a roaming patrol of Lindens around the grid... bugs seem to be scared of them.

Seriously, if I can help with this, let me know.