java.io.IOException: Server returned HTTP response code: 503 for URL in tomcat 6

I have production system which is having 3 servers on each of them tomcat6, apache are deployed. Requests are coming from Apache to tomcat6. In one of them I have tomcat5.5 and ejabbar. The chat requests are going from tomcat6 (of all 3 servers) to Apache of the server where the ejabbar is deployed and then it is going to the tomcat5.5 and then from there it moves to the ejabbar. Db used for the application is MySQL.
The problem I am facing is, after sudden time the application gets down for 10/20 seconds and then automatically it returns back to the normal state. At this point of time I can see the tomcat6 of all 3 servers are having java.io.IOException: Server returned HTTP response code: 503 for URL: <url for the chat server's apache> (The chat requests are coming for showing the online users in every 30 seconds and a polling for the user messages in every 10 seconds)
At this time I can see the db connections are growing at high speed, but from DB perspective didn't find any such major issue. The DB is used for several application along with my one.
Anyone having idea about this will be appreciated!!

"Amitava Gh", please check your private messages regarding an important administrative matter.