What do you mean by overloaded?
High CPU usage?
The hosts will be seen as disconnected from the vCenter when the vCenter misses the heartbeats from the hosts.
The hosts send hearbeats every 10 seconds and vCenter has a 60 seconds window to receive the heartbeat. So when the vCenter doesn't react sometimes because it's overloaded, there should be enough time to receive the heartbeat.
It's also possible to increase the heartbeat timeout:
http://kb.vmware.com/kb/1005757
