Sean Lumley has written an excellent blog article detailing how to debug pages that fail under load: http://blogs.msdn.com/slumley/pages/debugging-errors-in-load-test.aspx 

We have this requirement quite a bit in our performance and scale lab sessions when apps start to be put under stress that force different (and mostly unexpected) results from the time when they were recorded. This will be a great set of steps to help find the root cause.