This was fixed in the 2010 release. With some changes in the Windows OS, v2008 (and some early releases of v2010) would not be able to get past a certain number of items (e.g., ~150). With the current release of v2010, I don't believe we have seen another instance of this issue occur. I do think you can roll back to XP as a solution for v2008.33 (i.e., as opposed to slicing up your experiment into two chunks).