Hmm is your ESXi host up-to-date with the latest patches? Also, if the issue occurs again could you try posting ESXtop with expanded VM ('e' key, navigate to the VM and press enter) - maybe there is a world in the group which has some sort of resource leak which can cause locking up and misbehaving.
Also /vmfs/volumes/<vmname>/vmware.log, vmkernel.log and vmkwarning.log from /var/log would be very helpful in the time of occurrence.