P.S. I'm really, really, really happy that you've narrowed the problem down to a garbage collection issue. Implementing Arc lists with mpair's seemed to fix the queue bug, but I had no proof. I was still possible that I had merely moved data structures around in memory or whatever enough so that the bug simply wasn't manifesting by running the queue test. Now that we know that it's a GC problem, I have much more confidence that the mpair implementation is in fact one way to fix the bug.
