Call the ISP to rule that out as mentioned. I would disable IPv6 since you really don't need it in most cases.
After talking with the ISP, you may want to take your router out of the mix to see if it may be causing the issue. Also, within your router you should be able to verify the connection from the router to the ISP by IP address and a possible lease time. I would look there when you have the issue to make sure everything is correct. You should be able to release/renew at the router also.