I dont run 9.3, so I cant be of much help other than to say people are going to want a lot more information in order to help you out. They'll want to see system config, specs, etc.
I have a fresh install of 9.3 presenting a default iscsi (no chap or other) to ESXi 5. It reconnects after a reboot for me. Start looking through your ESX and freenas logs.