Debug advice when postgres connection maxing out

Server: OS X 10.5
PostgreSQL version: 8.3

We experience this connection maxing out once in the full moon.
The request from client reaches to the server but client never receive response back.
The queries are very simple update on one record or select one record using primary key (checked current_query from pg_stat_activity).
Once this started, I normally disconnect all the client (quit client programs) however, the processes don't die on postgres server.
So I tried to quit PostgreSQL server using pg_ctl, no response. The only way to terminate is to restart the server (I will try kill -s INT next time.. i didn't know better).
Is there anything you can tell me from this symptoms or anything I can do to generate useful log to analyze?

Thank you very much for your time in advance.