concurrency problem

Dear group,
    Let me explain my issue.
       We use
              Database      - postgresql-8.1
              JDBC Driver  - postgresql-8.1-407.jdbc3.jar
              Java             - jdk1.5
              The default transaction isolation level is - Read
Committed
              Auto Commit is false
    In our application we used a single connection object. We open the
connection in the MDI form and close it only when the MDI closes ,
simply when the application closes.
    I give a insert statment like
             INSERT INTO rcp_patient_visit_monitor (
                   entry_no, patient_id, visit_date, is_newpatient,
visit_type, is_medical,
                   is_review, is_labtest, is_scan, is_scopy, is_xray,
weight, height)
                   VALUES ((SELECT coalesce(max(entry_no)+1, 1) FROM
rcp_patient_visit_monitor),?,current_timestamp,?,?,?,?,?,?,?,?,?,?)

    The point to note here is the select statement which gets the max
entry_no and adds one to it and save the new value. entry_no is the
primary key of the above table.

   Now i run the same program (different instance) from two systems,
save the form simultaneously, only one entry is saved, in the other
system the error says - duplicate key violates.

   If i use the transaction level - Serializable - again one entry is
saved. Only on closing this application (closing the connection) the
application running in other system is getting saved.

   If i lock the table and create a transaction - by sending the
commands
         con.createStatement().executeUpdate("begin");
         con.createStatement().executeUpdate("lock table
rcp_patient_visit_monitor");
         int rows = psSave.executeUpdate();
         con.createStatement().executeUpdate("commit");

   The form in one system is saved, in another system an error says - '
Deadlock detected  .....'

    When i test the above said commands in dbvisualizer from two
different systems , it works, but here it does not. why.

    how to solve this concurrency problem.
    Also i want to know if a table is locked from jdbc.

Thanks in advance,
--
Sathish Kumar.S
SpireTEK