Messages merging with other messages in Windows Live Mail and deleting original message

Every so often one of my folders in Live Mail corrupts. All of the messages are there by subject but when you open select emails the message is gone and the message from another email in the folder is in it's place. This has happened a number of times and I end up deleting the folder and all of it's messages to stop it from spreading. I tried a repair on Essentials and my antivirus is up to date. Any thoughts???