Dpws based web service fails randomly

I have created a web service, and am testing performance on a Fex Raptor board. I call the web service continuously, it does nothing but return, and I get the following error. The error is random.

Exception System.NullReferenceException - CLR_E_NULL_REFERENCE (13)

#### Message: 
#### System.Net.OutputNetworkStreamWrapper::Flush [IP: 0014] ####
#### System.Net.HttpListenerResponse::System.IDisposable.Dispose [IP: 0018] ####

A first chance exception of type ‘System.NullReferenceException’ occurred in System.Http.dll
An unhandled exception of type ‘System.NullReferenceException’ occurred in System.Http.dll

Has anyone had a similar experience.

Thanks
Vicente

Post the code you’re using.