Ubuntu :: Tabs Start "spinning" - Possible OS Reinstall?

I have a relatively new dual core 64 bit dual install Win XP/10.04 system. At first I was able to run quite a number of browser (Chrome and Firefox) windows simultaneously in different desktops without much in the way of problems save the occasional reboot every few days to let the machine clear it's head. Lately when I reboot, it runs fine for a few (1-6) hours, but then one or more of a few of the tabs start "spinning" in the tab showing that they are refreshing and a popup comes up saying that the program isn't responding, and asking whether I want to wait or force close.

If I choose wait, and go to those tabs, and hit the stop button, it changes to a refresh button but the tab still shows that it is attempting to connect/refresh/whatever it is doing, and the popup returns every few seconds. I.e., it is ignoring the "Stop". I am now only running a few browser windows which is a much lighter load than the multiple windows before. I have reinstalled both Firefox and Chrome without any improvement. There doesn't appear to be any pattern to the ones that start doing this, but once started any new tab will often fall into the same problem. when I boot up, just after selecting the OS I want, a message flashes up (white print on black)saying: What is causing this behavior? What can I do to solve it? What information would be helpful to you? Do I need to reinstall Ubuntu?