It seems that if you’re running a host that’s running ESX 5.0, there’s a bug with FreeBSD VM’s which causes the internal timer to just freeze. It’s fixed by a patch released for ESX version 5.0, or upgrading to 5.1.
This came up with a FreeBSD 8.3 guest running our Statseeker instance where the time just up and locked, then refused to go forward. A reboot would fix it, then it would lock up again at some arbitrary point in the future.
The fix from Statseeker was to disable NTP and we’re on a “hope for the best” plan at the moment until we can get the host upgraded to 5.1 or the patch applied.