I am an experienced Joomla user but I've never seen something like this during my 4 years' Joomla life. After I change anything on the back end, such as article's title, or menu item's title, the changes won't immediately show up on the front-end until about 30 minutes later. For some of my computers/browsers it changes a day after, and some hasn't changed at all yet even though I made the change 2 days ago.
At first I thought this was a problem of cache, but I've done clearing cache for browser and back-end, I even use some machine/browser that has never opened the page, the problem is still there. Besides I don't think things like menu item title or article title get into cache. So what could possibly be the cause of problem?