In the interest of setting the right foundation for communication it's important to realize that my software designs skills/understanding are intermediate and do have limitations. That having been said I will describe my situation.
Basic Description & Developement Environment:
I have designed an ASP.NET (.NET Framework 4.x) webservice using Visual Studio 2010 that receives an xml document through a SOAP call made by the end user. The XML is then processed by 10-15 different classes. These classes retrieve data from 3rd party information providers, credit bureaus, OFAC repositories ect.... and subsequently parse the results. The final process simply constructs an XML document of the results and returns it to the end user.
Occassionally when two users make requests at the same time their XML results become "mixed". That is to say that user A may have data contained in the XML that belongs to the request of user B. It appears to be random and not consistently repeatable.
My inital thought was that I have not properly designed the classes to be thread safe/isolated however after much code review I don't believe that to be the case. Unfortunately all my attemps (limited knowledge not withstanding) to correct the problem failed and consequently I am stuck.