Message Threads Missing from Single Account

[iswope]

We have an Open Source Zimbra 6.0.7 install with around 70 accounts setup. We are having intermittent issues with messages suddenly disappearing from the mailbox of a single user. As far as I know this is not affecting any other users. Looking through mailbox.log shows no deletes or moves for the messages, they simply just cease to exist anymore. I've actually sat in front of the computer and watched them disappear in front of my eyes.

We tried a number of things to try to sort this out. We upgraded from our old 5.x.x install to the latest 6.x.x version thinking that might sort out the db. We doubled the ram for the server. We even backed up all of the mail for this account and then deleted it and recreated the account as a brand new setup and still about once every other day or so an entire message thread will just disappear without a trace.

I'm sort of at the end of my rope about this and am not really sure where to proceed further. I've searched for other threads related to this but haven't found anything. Any help would be much appreciated.