I face a similar problem. My research on this issue till now has just lead to a discovery of a property called "DebugSessionCrossover" in Websphere. This is a custom property which needs to be set to "true" in the webcontainer custom properties. This will help in detecting a session crossover(which is normally not a very consistent problem to reproduce). It provides a log in the systemout.log with the stack trace of the thread which is causing the fault.
I have not solved the problem yet, but this could be the first step towards the solution. Anyone who has a tip or solution, please help me.
Thanks.