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