Abt linux web server

Dear all,
I have a linux web server at my setup ,tomcat apache installed on it.
Now a days we are facing a problem that the sites on this server are not opening,it shows following error,’service tempararily unavailable.when i restart the httpd & apache service on the server .than itis working ok.but this is not a permanenet solution .the problem will exist after 2-3 days .
suggest me pls.

Deep