I have the same problem. After contacting my ISP and doing some troubleshooting their hosting provider seems to be blacklisting a bit to many IP's.
With another customer, we've tracked the problem to his ISP DNS.
Please try to set your DNS to Google's DNA:
Primary: 8.8.8.8
Alternate: 8.8.4.4
Indeed, we're using Shopify e-commerce platform.