I am using RB411U and noticed that the router is terminating the PPP session every 4 hours and re-establishing it straight away.Is there a timer somewhere controlling this behavior?

Wild guess: are you using RADIUS? Is the server sending the Session-Timeout attribute?

yes, I am using Radius server. I'm getting following log messages from RouterBoard and Radius server:
On RouterBoard
PPP connection terminating (hang up) then re-authenticating.
On server:
21/09/10 15:46:31 cdmat2 Connect Request
21/09/10 15:46:31 cdmat2 172.29.7.64 Connect Accept
21/09/10 15:46:31 cdmat2 172.29.7.64 Accounting - Start
21/09/10 16:46:31 cdmat2 172.29.7.64 Accounting - Alive
21/09/10 17:46:31 cdmat2 172.29.7.64 Accounting - Alive
21/09/10 18:46:32 cdmat2 172.29.7.64 Accounting - Alive
21/09/10 19:46:32 cdmat2 172.29.7.64 Accounting - Alive
21/09/10 19:47:11 cdmat2 172.29.7.64 Accounting - Stop
21/09/10 19:47:18 cdmat2 Connect Request
21/09/10 19:47:18 cdmat2 172.29.7.64 Connect Accept
21/09/10 19:47:18 cdmat2 172.29.7.64 Accounting - Start
21/09/10 20:47:18 cdmat2 172.29.7.64 Accounting - Alive

Any suggestion??

Add a logging target with a topic of 'radius' and an action of 'memory' and see what attributes are sent, or watch on the server. If a Session-Timeout attribute is sent the value will counted down from as seconds and the session will be terminated when the counter hits zero.

The four hour reset did not re occur last night but there was a period of half an hour where the PPP session kept dropping and the other router (other brand) next to this one was stable over the same period. I added a logging target with a topic of 'radius' and an action of 'memory' and got the log informations (please see attached file).
Someone please let me know what might be the problem..
Thanks

problem solved
RouterBoard has been stable for 24 hours.

Hi there i am getting the same issue how did you fix this

how did you solve the problem
it still comes sometimes not regularly