HttpExtensionProc::jk_isapi_plugin.c (2328): service() failed with http error 503

We are facing two problem using isapi redirector on a virtual machine.

Every couple minutes the log shows http error 503:

...
[Sun Oct 14 14:25:02.465 2012] [8428:7856] [error] ajp_send_request::jk_ajp_common.c     (1662): (application) failed sending request on a fresh connection (recoverable), socket -1   (errno=54)
[Sun Oct 14 14:25:02.465 2012] [8428:7856] [error] ajp_service::jk_ajp_common.c (2643): (application) connecting to tomcat failed.
[Sun Oct 14 14:25:02.465 2012] [8428:7856] [error] HttpExtensionProc::jk_isapi_plugin.c (2328): service() failed with http error 503
...

The log is filled with WriteClient errors

...
[Sun Oct 14 14:24:53.559 2012] [8428:1300] [error]  isapi_write_client::jk_isapi_plugin.c (1286): WriteClient failed with 10054 (0x00002746)
[Sun Oct 14 14:24:54.121 2012] [2424:7324] [error] isapi_write_client::jk_isapi_plugin.c (1286): WriteClient failed with 10054 (0x00002746)
[Sun Oct 14 14:24:54.199 2012] [8648:10692] [error] isapi_write_client::jk_isapi_plugin.c (1286): WriteClient failed with 10054 (0x00002746)
...

We are using Tomcat 5.5.26 which is connected to IIS 6.0 (Windows 2003 Server) using Jakarta connector(isapi_redirector.dll) on VMvare.

The file worker.properties is:

worker.list=application
worker.maintain=60

worker.application.type=ajp13

worker.application.host=127.0.0.1
worker.application.port=8009

connection_pool_size=400
connection_pool_minsize=200
connection_pool_timeout=600

server.xml contains:

...
<Connector port="8009" 
           emptySessionPath="true"
           enableLookups="false"
           redirectPort="8443"
           protocol="AJP/1.3"
           minSpareThreads="20"
           maxSpareThreads="100"
           maxThreads="400"
           bufferSize="8192"
           connectionTimeout="600000" 
           />
...