Since our DB server is behind a firewall, the zabbix agent is configured as active check mode sending out data via http proxy server (there is a WideCap 1.5 Proxifier program to convert the traffic at port 8080 to port 10051). It works well but zabbix agent memory usage continues to increase from 9820K to 51544K in around 6 hours, then it finally crashes due to out of memory exception. The following is the problem signature recorded by Windows server:

Problem signature:
  Problem Event Name:	APPCRASH
  Application Name:	zabbix_agentd.exe
  Application Version:	0.0.0.0
  Application Timestamp:	4fba4afe
  Fault Module Name:	KERNELBASE.dll
  Fault Module Version:	6.1.7600.16850
  Fault Module Timestamp:	4e211485
  Exception Code:	0eedfade
  Exception Offset:	0000b9bc
  OS Version:	6.1.7600.2.0.0.272.7
  Locale ID:	1033
  Additional Information 1:	0a9e
  Additional Information 2:	0a9e372d3b4ad19135b953a78882e789
  Additional Information 3:	0a9e
  Additional Information 4:	0a9e372d3b4ad19135b953a78882e789

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The extract of zabbix agent log just before crash is also attached here for your reference.