We have 400 Mbps symmetric with Hotwire, and whenever I do a speedtest, I get at least that much, sometimes it bursts to 600. But then there's something wrong

We have 400 Mbps symmetric with Hotwire, and whenever I do a speedtest, I get at least that much, sometimes it bursts to 600. But then there's something wrong with their peering or something. Youtube videos randomly buffer even at 1080p. Downloads randomly drop from 400 Mbps to 10. Sometimes the problem lasts an evening, and then goes away. I have no idea.