If your vCenter Server is behind a NAT the hosts will drop out after about a minute. To fix this problem you must add a port forward on the router to relay UDP/902 back to the vCenter server and configure the Virtual Center agent on the host to point to the external NAT IP address.
VMware Reference: http://kb.vmware.com/selfservice/mysupport/viewdocument.do?externalId=1010652
IP Tables Configuration:
iptables -t nat -A PREROUTING -i {WAN_INTERFACE} -p udp --dport 902 -j DNAT --to-destination {VCENTER_IP}:902