PQExec returns null question

Our OS X application uses PostgreSQL as a back end through pqlib. Some of our customers are reporting occasional
PGRES_FATAL_ERRORs,but PQresultErrorMessage returns a blank string. I have looked at the sources, and as far as I can
tell,this condition can only occur when PQExec returns a null. The docs say that this shouldn’t happen except in out of
memoryconditions. I am not using any Copy actions. 

I have two broad questions. 1) What can I do to prevent this from happening? 2) If it does happen, how can I remedy it?

I have increased the shared memory, and that seems to have helped somewhat, but it still occurs. In our experience,
oncethis starts happening, it continues for every subsequent query.  WE have found that restarting the machine will
remedyit for a time, but is there a less drastic action we can take that will also remedy it? Perhaps stopping and
startingPostgreSQL? Or are system resources being consumed to the level that a system restart is required? 

Many Thanks,
Don Carlile