Some hosts configure their services with default filtering of requests from certain user IP addresses. When you use Fasterize, queries to your origin all come from Fasterize server IP addresses, which can cause certain Fasterize IP addresses to hang. The consequence is that access to the pages randomly fails, it depends on the IP address by which the request arrives at your origin, which can be an address blocked or not. 

This can be corrected by configuring your original servers to use the real IP address of users, not Fasterize, using the HTTP header  X-Forwarded-For as explained in  this article .