I had the same issue before with a website. I still do have the issue here and there. I'm not sure if it is the ISP or the server. Though another website I had a issue with just like this one. They had done something to the DNS and it worked perfectly. So I can't say if it really "was" the DNS issue or my ISP. Best way to test it is mabye not using the router for the day and see if the PC connected to the WAN/modem shows the same issues.