When multithreading on the Xbox 360, here are a few things to keep in mind.
Affinity
In multithreaded programming, programmers often lack the ability to specify the Processor Affinity of a thread. Even when API calls are available to specify a processor, it is usually ignored. The reason for this is that modern operating systems have many processes running, each with many threads. The operating system zealously manages processor affinity in order to maximum some metric.