I was going to suggest a different browser, but if it's happening with two already I'm not sure what difference a third would make. Can't hurt though, www.opera.com for Opera (it's free).
If that's similarly affected then I can really only think of a couple of other things - the operating system, some other piece of software or the router. Router's probably simple to rule out, just reboot it and check any parental or content control settings if there are any.
From there the most likely candidate is some other piece of software that's running, do you have any ad blocking software or firewall software?
I guess there's an outside chance the harddrive's full, although I'm sure you'd have noticed other things if it was.