Postgresql/PHP backend error

I have PHP 4.06, Postgres7.1 and Apache on a Solaris server. Most of the time all works as expected. Occasionally I get an database error: Database error There was a database error when accessing Database zwg: pqReadData() -- backend closed the channel unexpectedly. This probably means the backend terminated abnormally before or while processing the request. (while evaluating: SELECT serializedobject FROM objects WHERE name='Zurich') [This error page is generated by my application; it displays the error reported by Postgresql and the SQL that caused the error]. The error is not caused by my SQL syntax, since this same code executes without a problem at other times. Has anyone else seen an error like this? Where do I start looking for the cause? Which backend is terminating abnormally (Postgresql or PHP?) and why should it do that? Nigel Gilbert