After upgrade from 2.0.6 to 2.2.3 our zabbix-system have full queues. In details view we see that there are items which have a 3600s intervall stuck for about a hour in queues. It seems that proxy/server ask for these items but don't wait for that or Timeout is to short.
After a hour all items vanished from queue, so we think the answer from the first round arrived.

On our proxies we start 50 JavaPollers. On JavaGateway we start also 50 JavaPollers. So there is no unbalanced situation we think.