I would recommend that you do several things:

#1 - update to latest git

#2 - rotate logs

#3 - enable uuid logging (see logfile.conf.xml - param name "uuid")

#4 - reproduce the symptom with a single call (if possible)

#5 - pastebin the log for the uuid in question and link to it in this thread

From there hopefully we'll get a clue as to what is happening.

-MC