Exchange 2010 sp1 Web mail OWA error "The server operation timed out, please again later" or sometimes "Server busy"

Hi,

We have Exchange 2010 sp1, All the users are getting below error when they try to access the mail through OWA externally. Internal OWA is working fine.

Error "The server operation timed out, please again later" or sometimes "Server busy".

The error we are getting from the browsers (IE 8,9; Mozilla; Chrome etc..)

Can some please help us on this.