This gives me an idea.  Are you using threads and generators, perhaps?  Could this be related to Bug #1579370?  http://python.org/sf/1579370  You can find a short discussion on it from python-dev around Jan 22-23.  Martin checked in rev 53531 on trunk.  This change might be in 2.5.1, I don't remember when it came out.  Try rebuilding Python with this patch installed and see if it fixes your problem.