connection failure

What does the message below mean?

server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

This message appears sometimes after i send a 'BEGIN' to the server and
the processor load of the application goes up to 99%. This happens
usually when the application starts and several threads start sending
queries to save the changes since the last stop. 

The application its locking mechanism (we are aware of that libpq
doesn't have interlocking) has worked fine in the past. This issue has
became serious since we moved to the latest debian lenny distribution
and started to use libpq5.

OS : Debian linux Lenny kernel version 2.6.26-2-686
libpq : libpq5 8.3.7-0lenny

Thank you in advance
Attila Miklosi