Moop

I've been having an ongoing problem with my internet speed dropping off, and could use some help making sense of it. Essentially, I notice that occasionally (& regularly) the speed drops off a cliff, which typically results in streams that I'm watching in buffering. When I run a speed test, I pretty much always see the fast speed advertised with my service. So I tried out testmy.net, running an automatic download test every 5 minutes for 8 hours. The resulting graph looks like a saw blade. Every 3 or 4 intervals, the speed is very slow: https://testmy.net/quickstats/MooseP?type=user_name&q=MooseP&limit=100 ?t=u&d=04-29-2020+%2F+04-30-2020&l=100&q=MooseP's Speed Test Results This is on a wired desktop, and I can't think of anything else in my house that would be causing this. Does this look like my ISP throttling me?