Postgresql 9 connection problem

I have very strange problem with my application, upon occasions I get
following error when connecting to postgres. This happens without any
pattern, often happens when I run some extensive tests in my application.

Unable to connect to PostgreSQL server: server closed the connection
unexpectedly This probably means the server terminated abnormally before or
while processing the request.

Postgres logs shows no abnormalities. Moreover, I can easily connect to the
psql console and execute query at that period.

My Postgres is 9.0.1, running on CentOS 5.

Any hints ?