Sometimes we have poor internet connection on zabbix server, and then some monitored agents (versions 1.4.x, 1.6.x, 1.8.x) hang. For example, I see in dashboard:
Server server1 is unreachable
Server server2 is unreachable
Server server3 is unreachable

If I try to telnet to server1 zabbix port, it does not answer:
telnet server1 10050
Trying 82.144...
Connected to server1.
Escape character is '^]'.
ddd

And it hangs here. If I issue 'service zabbix-agent restart' on a monitored client, I can telnet successfully:
telnet server1 10050
Trying 82.144...
Connected to server1.
Escape character is '^]'.
ddd

ZBXDZBX_NOTSUPPORTEDConnection closed by foreign host.

So I have to login to every hanged client and restart zabbix service. It happens with both Linux and Windows agents, versions 1.4-1.8.
Zabbix server version is 1.8.1, but it used to happen with 1.6.x too.