too busy error in my wcf service

Question

I create a webservice in wcf ,with these config :

<?xml version="1.0"?>
<configuration>

  <appSettings>
    <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
  </appSettings>
  <system.web>
    <compilation debug="true" targetFramework="4.5.2" />
    <httpRuntime targetFramework="4.5.2"/>
    <authentication mode="None" />
  </system.web>
  <system.serviceModel>
    <behaviors>
      <serviceBehaviors>
        <behavior>
          <serviceMetadata httpGetEnabled="true" httpsGetEnabled="true" />
          <serviceDebug includeExceptionDetailInFaults="false" />
        </behavior>
      </serviceBehaviors>
      <endpointBehaviors>
        <behavior name="web">
          <webHttp/>
        </behavior>
      </endpointBehaviors>
    </behaviors>

    <services>

      <service name="lpz.Application.ServiceImplement.InquiryService" >
        <endpoint address=""
                  binding="webHttpBinding"
                  contract="lpz.Domain.Service.IInquiryService"
                  behaviorConfiguration="web"/>

      </service>
    </services>

    <protocolMapping>
      <!--<add binding="basicHttpsBinding" scheme="https" />-->
      <add binding="webHttpBinding" scheme="https"/>

    </protocolMapping>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true" multipleSiteBindingsEnabled="true" />
  </system.serviceModel>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Methods" value="GET, POST,PUT,DELETE" />
      </customHeaders>
    </httpProtocol>
    <modules runAllManagedModulesForAllRequests="true"/>

    <directoryBrowse enabled="true"/>
  </system.webServer>

</configuration>

I decided to test my service using j_meter ,a simulator for create request ,so i create a fake request to call my service with 20 thread .so i did that ,but my service working for 4 minute and after that i get this page : and after 4 minutes it works again and this process is repeated.why ?