My money's on it being an overloaded neighborhood node.

The backbone of the internet usually lives at datacenters with redundant power/generators/etc. Even the little ISP that services my office has ~24-48 hours' worth of backup battery power on-site and generators.

Now, what DOES affect my network speeds is rain fade due to the microwave (and higher) links used by my ISP. At one of our remote locations you can see the signal strength actually vary with the tide.