Thread suddenly stops in NT Service

I'm running an NT service with a thread. The thread performs a
calculation every so often and appends the number to a text file.
Every so often the thread just stops - or so it seems. The text file
stops getting numbers added to it but the service is still running and
no errors have been thrown. I'm quite sure that the code would catch
an error and post it to the event viewer if one were arising.

The only other relevant points I can think of are that the thread runs
a while 1: loop and is set to run as a Daemon. The service waits only
for the stop command. It doesn't seem to be operating system specific
(happens on both XP and 2000). It doesn't seem to be file size
specific, it stops at various file sizes ranging from 5K - 50K. It
doesn't seem to be time specific - sometimes it'll stop after about a
day. Sometimes it'll stop after two days.

Any help would be great. Thx.

Matt