Chats are not going through - and then all coming through at once after 10+ minutes!

Hi, we are having a problem on our realm with severely delayed chats. Everyone will be typing in chat, and nothing coming through, sometimes for well over ten minutes - and then suddenly all the chats will come through at once. Sometimes after players have gone offline! It is NOT our chat privacy settings, as we all have them open to everyone, and the chats are coming through eventually in a huge bulk, but severely delayed. Please can someone help to fix this?