Vaadin application hangs on browser after 30 minutes

My application conatins 5 tabs , the first tab keep on send requests to server to refresh the tab,i do not face problem when i open few sessions, when i open more than 25 sessions and keep the application running for more than 30 minutes , one or two sessions got hang in browser,

i analysed with jdb tool of Jboss process in solaris , the http-client-thread waits for object monitor. and this causes more usage of cpu also .Is it because of synchoronized block in vaadin framework ?
please help me to resolve this issue