Gravestones keep disappearing from my game.  This is from a residential lot, with an Addams Family type couple and 9 graves, in my own neighborhood.  The character data remains, so I can use the modthesims assignable gravestone to give the sim a new tombstone, then resurrect him/her and kill them again to get their own gravestone.  I do that with one grave though, play for a half hour, and then glance over at the graveyard and notice that another one is gone. 

I have University and Nightlife, and the Nightlife patch.  I saw the post here about the tombstones of ghosts who are out when the patch is applied disappearing, but the most recent tombstones disappeared today, while I was playing, a week after patching.  I also don't have the disappearing character data mentioned in that post and it's not in Pleasantville.  I installed the deathfix after the first grave disappeared yesterday, but two more went after that.  My neighborhood only has 474 people in it, and the notownieregen fix is installed.  I don't transfer in anyone from other neighborhoods and I only have 47 downloads (all from here or Maxis, except the swinger's bed, teenwoohoo, and the mts buyable tombstone).  I hope that's enough info and that I'm not asking a stupid question.  Please help me keep my ghosties.

I had a problem like this before Nightlife.  Mortimer's grave was constantly vanishing from the Goth cemetery and everytime it went, his relationships went with it and I had to keep putting them back.  I think maybe the Goth women ghosts were responsible, because they were forever jumping in and out of both his grave and Bella's and as they were the only platinum ones, I think they were jealous.  I took a look at their files in SimPE and they all wanted to turn Mortimer into a zombie.

One word of advice - if you resurrect someone so you can kill them again to get a gravestone (incidentally, you can do that even if they have no gravestone, as long as they have character data.  You don't need to use an assignable one to resurrect them) make sure you make them Linked in SimPE before you bring them back.  If not, they will act as if they are new Sims from that point on and won't recognise their family members, thereby being very noisy ghosts instead of quiet ones.  I accidentally deleted Cornelia Goth's tombstone along with some weeds and even though she was only alive for a few seconds before I killed her again, she no longer accepted Alexander's sons and kept screaming that they weren't part of the family.

Yup, you're right, I just watched the ghosts jumping into each other's graves and the gravestones hopping around and then disappearing.  I never thought I'd play a video game to stare at headstones for half an hour, blah.  I worry that even on a community lot they might jump around like that, the loss would probably just be slower because there wouldn't be sims living there 24-7.  I think the only safe place for gravestones might be the inventory 

Maria, I don't think it would happen on a community lot because community lots aren't saved when you exit them with a sim. The states of the sims are saved, but I don't know if it would apply to ghosts since they're tied to the objects, and the objects are set back to the state they were in when you last saved the lot in build mode when you have your controllable sims exit the lot for another or home.

Twojeffs' ghost hack prevented the ghosts from haunting other ghosts' graves. Apparently this is (a|the) cause of disappearing graves.

It has not been updated for NL, though.

i have disappearing tombstones too! on the Specter house. i thought i lost them when i was saving the game and some error occurred. now Olive is gone forever!! sob!!

Ok this may be totally unrelated but, not so long ago I had a mailbox dissapearing off my lot.  I never saved and when I restarted the game the mailbox was there.  I continued getting my sim to do the same things as the previous occasions with my eyes on the letterbox so I could define what was causing it to dissapear.  It didn't take long and was reproducable several times over.  Everytime my sim sat down and started to read a book from the cinderblocks bookcase (the cherapo one with planks and blocks) the mailbox dissapeared.  Deleted said bookcase and replaced it with a more expensive one and no more problems. 
If you are finding its happening again and again keep tabs on what your sims are doing at the exact time your gravestone dissapears, could be it's the same sort of glitch.

One of my sims had a fear that his grandfather's tombstone would be kicked/destroyed.  I haven't had this problem but I have noticed that the headstones sometimes revolve180^o