I've been playing with my new wireless card that I installed on an old PIII PC running Ubuntu 9.04. I connected up to a unsecured WiFi router and was surfing the web. Suddenly I noticed that my hard drive lite was continually on and the machine locked up. I rebooted and connected to a different unsecured site and no lockup occurred. Then I went back to the original site and the same problem occurred. Is that site using some type denial-of-use algorithm instead of WEP or WPA?
I have experienced this kind of thing while at a friend's place and we concluded that there was some issue with the router we had accessed -- perhaps deliberate. The symptom sounds similar to yours.
But this was a few years ago when few people round here secured their routers and, I rather doubt that back then anyone had the knowhow to ambush people that way.
Anyway the days when you could borrow other peoples internet have long passed in most places as ISP's routinely supply routers already secured. Incidentally, in most jurisdictions it is illegal to use someone's internet connection without permission.