Form-Based Login

[Rich Knight]

Hi,
I'm attempting a simple Form-Based login to authenticate a user and give access to my site. I authenticate their username and password against a database and based on the role they have (also stored in the database) I grant them access. I'm using Tomcat 4's JDBCRealm to authenticate.
Unfortunately my results are very inconsistent. Sometimes I'm able to login and sometimes I get 404(Resourse not found) errors. In the address bar I notice it reads: "<myApp>/j_security_check" when it should be forwarding it to "<myApp>/index.jsp". Also I sometimes get a 401 (Invalid Redirect) error. Below is the relevant code to my login page, taken straight from tutorials. Any help would be appreciated.
Thanks
RK

 
Login Page
<form method="POST" action="j_security_check">
Username: 
<input type="text" name="j_username">
Password: 
<input type="password" name="j_password">
<input type="submit">