I am experiencing intermittent errors upon invoking my web service method.

Exception Type: System.InvalidOperationException
Exception Source: System.Xml
Exception Message: There is an error in XML document (1, 290).
Exception Stack Trace:    at
System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader,
String encodingStyle, XmlDeserializationEvents events)
  at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader
xmlReader, XmlDeserializationEvents events)
  at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader
xmlReader)
  at
System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage
message, WebResponse response, Stream responseStream, Boolean asyncCall)
  at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String
methodName, Object[] parameters)

I suspect this means that IIS returned a HTML error instead of a well formed
SOAP response.  Is this correct?  What is the best way to track the real
source of this error?

.NET 1.1 client
.NET 1.1 server / Win2003 server

Thanks