The update is we are still working on this problem and we have looked at a lot of code.  But what we can't do at the present time is go back to the 6.6.7 build (which does not display this issue) and start adding stuff one-by-one on the path to 6.7.x until we hit something.  Then if we determine it's a kernel change, we face the task of bisecting the kernel.  The problem with this approach it that it's extremely time-consuming, especially when each 'run' might take several hours or even days to