Turning on Motion Blur will cause Krakatoa to get the Velocity channel from the Fume sim. Is there anything funky about your Fume velocity channel? I’ve heard that Fume doesn’t handle velocities very well, are you sure that it isn’t just thrashing around in the page file because you spilled out of main memory? You can check using Process Explorer or Task Manager to see what kind of memory usage is reported for Max when these hangs are occurring.
Let me know what you find out.