I don't know of a permanent solution for your problem. but a good workaround would be to edit your resolv.conf file to point to a different dns server that your ISP supports. Alternatively, you can also point to public root dns servers from either ICANN or OSRC. I personally run my own dns server on my laptop and I've noticed far fewer slowdowns compared to using my ISP's dns server. I don't know of a good troubleshooting method for getting good rates on the internet other than to ping several dns servers for the fastest response times, and then put the 2 fastest in the resolv.conf file.