Libpq Asynchronous Command Processing

Hello,
I am migrating a client/server application from Debian Sarge to Debian 5.0 and I am finding problems with the client
application.The facts are the following: 
->The client application is an interface to a Postgresql DB so it uses libpq.
->The client application compiles properly in both Debian Sarge and Debian 5.0.
->The client application employs Libpq asynchronous command processing.
->The pseudo.code for the queries is the following:
PQconnectdb
PQsendQuery
PQflush
loop{
PQconsumeInput
PQisBusy
if not busy PQgetResult and leave
}
->The new DB server is postgresql 8.3.
->The old DB server is postgresql 7.4
->I am using the same SQL script to create the DB.
And the problems I am finding are the following:
->Queries from the client to the new DB server take a lot of time.
->Queries from the client to the old DB server are fast.
->The same query takes 150 secs in one case an 1 sec in the other case.

¿Any ideas regarding the origin of this strange behaviour?¿Could it be the configuration of the new DB?
Thanks in advance.