First, clustering problems are hard to solve using community forum as setup required to produce your issue is time consuming and not easy. This is when you should buy support contract. In your case, it fits more because you said it's "very urgent".
You did not mention which portal version you are using. If you heed to my previous suggestion then you should be using JBoss Enterprise Portal Plantform (EPP) 4.3. On community side, use Portal 2.7.2 with AS 4.2.3. These versions have lots of clustering fixes.
I am using jboss portal 2.6.2.
Can you expalin me why this behavior is there?