I've got a few more than 3000 messages and I don't have any problem sending. I think at some point you need to take the advice of others, flipalong posted a link that I think will, at best, fix your issue, at worst, do absolutely nothing. Give it a try. From your description it sounds like your database is corrupt - you can't make it any worse at this point.

If you are really paranoid about losing messages, install glogarchive and back it all up, or just copy the database file elsewhere before you do anything.