[racket] handin-server hanging

Has anyone had any problems with their handin-server hanging?

Twice recently we've had our handin-server seemingly hang.  The last
log message on both hangs was that it was "Cleaning up all submission
directories".  I've added new log messages to determine if it ever
finishes cleaning the submission directories should this happen again,
but I haven't been able to reproduce the hang easily.  We have around
400 students in the class using the handin-server, so the number of
submissions is adding up quickly.  The server is running on a host
where the submission directories are served over NFS, which does seem
to be slow in some cases, and could be a source of Racket hanging for
some reason.

The process wouldn't respond to a Ctrl+C interrupt on the terminal it
was running when it was hung; I had to send it a signal using kill to
get it to stop.

Anyone have any suggestions on where I should look for this?  I'll try
to post more information as soon as I get it.

Thanks,
-ryan