It could be a couple of things. Generally you will have Dynamic IP from your ISP. This changes every now and then. It could be that your IP gets it's IP blocked for accessing the service, then when the IP changes then it is not blocked for a while until they see you accessing the service and then block it again.

It could be in the way the ISP is routing the connection from where you live, sometimes it takes a rout that is blocked somewhere down stream and other times it is not because it is taking another route that is not blocked.

If you constantly have this issue, I would recommend setting up a separate VPN router since you already have a VPN services and route the NFPS stuff through it so you do not have to deal with the intermittent issue.