zimbra-4.5.10 mysql always failing

[shinacalypse]

hi.. im running a distributed zimbra installation ( with 3 mtas and one mailbox store). Periodically i see that the mysql server stops running for no reason. Email still appears to be flowing during that period.. i out of fear just restart the zimbra services.
when i restart the zimbra services everything goes back to a working state for an hour or so and then i see in the logs ( watch zmcontrol status) that mysql is not running again.. what could be responsible for this, and what can i do to stop it?
The mailbox server (installed with everything but mta service) has 16GB of RAM, and we set mysql memory percent to 10, after varying it from 12 and 11 ( to keep it below or just at 2GB as recommended.

[uxbod]

Does anything show in your Log Files - Zimbra :: Wiki ? Also check under /opt/zimbra/db for a file _zmhostname_.err and see if MySQL is reporting any errors.

[shinacalypse]

Hi uxbod..
its strange.. but i see no errors in any of the logfiles.. ive been looking at my hostname.err file and there are no mysql errors generated.. would you like me to attach any of them? most of them are quite large..

Im tearing my hair out here.. it seems to die by the hour this morning.. and then it stays ok till the end of the day. There are about 5000 users.. and it IS 4.5.10... could this be load?