I have had this problem before and it has improved since I replaced the HDDs in the shared storage devices (iSCSI) I am using. However, today this happened again. I am wondering why I can't ping the host even though I know the host is powered on. I have to power cycle the host to get it online again.
I am not sure exactly how the heartbeat works but from what I've read, the hosts send out a heartbeat to vcenter server every 10 seconds. If there is no response in 60 seconds the host drops out of the cluster.
any ideas as to why the host is "not responding" to anything even a ping?