If you are having problems with one proxy, try a few different ones. I recommend using firefox with foxyproxy as it is very easy to change proxies that way. I have tried some proxy servers from proxy4free and they were pretty good. They also update regularly so keep trying different ones. And don't forget to change the port.

Proxy lists:
http://www.proxy4free.com/
http://www.publicproxyservers.com/page1.html
http://nntime.com/

And you can use Hotspot Shield instead of proxy servers.