Process hanging on Endselect

We have a process which needs to produce an average figure over the
last 16 qualifying weeks. The way it currently works is to select the
rows from the two relevant tables ordered by date descending, then in a
select loop check if the week qualifies and total the data. When 16
weeks have been processed it does an "Endselect" to leave the select
loop and carry on with the processing.

Occasionally this process appears to hang. IPM reports no locks and no
current SQL Query, and a VMS "Sho Process/Continuous" shows the process
in LEF state with no activity. If we kill the job and re-run it it
(normally) completes without any problem. Looking at the output, it
does not always hang on the same record.

Further investigation this morning (running the job in debug) suggests
that the job is hanging in the "ENDSELECT" statement. Other than
rewriting the job to not use select loops at all, does anyone have any
suggestions or insight into what is going on?