It may be the free memory is too low,kill some useless processes to get more.The default minimum free memory percentage is 5.You can change it in the config file like below:
<serviceHostingEnvironment aspNetCompatibilityEnabled="true" minFreeMemoryPercentageToActivateService="1"/>
If you set that percentage up to 99,than 'Undefined' error will be reported again again and again.And if it happen,you can open the wcf(.svc) file in IIS.Than will show you the error details.
Good luck!
The 'serviceHostingEnvironment' section format:
<serviceHostingEnvironment>
aspNetCompatibilityEnabled="Boolean"
minFreeMemoryPercentageToActivateService="Integer"
<baseAddressPrefixFilters>
<add prefix="string"/>
</baseAddressPrefixFilters>
<transportConfigurationTypes>
<add name="String"
transportConfigurationType="String" />
</transportConfigurationTypes>
</serviceHostingEnvironment>