pg_connect sometimes works sometimes not

Hello,

we migrated a php code from FreeBSD 5.x, PostgreSQL 8.x and php 4.x - to
the latest versions of these, keeping the configuration options. Now
pg_connect started to fail on irregular intervals for no obvious reason.
Before we had a problem with the number of connections, but we monitored
them and they are less than 10 (out of 100 available).

What other variable (which are sometimes there, sometimes not) reasons
there can be pg_connect to fail?

Thank you,
Iv