problem when two or more users access simultaneously?

Hi,

    I am building a site where users log in and access tables in
two diffrent schema.

I'm using PHP 4.3 and PostgreSQL 7.4 and I have problems
with involontary logouts since the users credentials cannot be verified
when reloading the main page of the application. Typically this occurs
when users are involved in similar activity, dealing with the same
tables.

All transactions are handled through the same postgres-user and are all
done through plpgsql-scripts.

I am yet not *sure* that the problem is with the database, I am
exploring different possibilities.

My questions are: Are there any known problems with concurrency and
if so are there any tricks or such to solve or bypass this problem?

Any takers?

regards
/Dag