I'd like to know this as well. I have fiber 300/300 from FiOS and I'm getting constant dips in quality to the point where it's kicked me off. The problem is, nothing I've done for troubleshooting has worked, even the things listed on this help page. I've been using the chrome browser on my PC, which is connected directly to Ethernet. Nothing else running, or anything else using the internet but me.
Folks, regarding your network connectivity issues, have a look at the jitter and latency value when performing a speedtest or something equivalent as those metrics are almost more important than your bandwith itself.
The way network connectivity works means that your ISP in order to not being sued can really provide you a full bandwith connectivity but yet have important connectivity, latency and jitter issue even on fiber connections because of plenty problems such as intermediate appliance made to snif and "secure" the network, DNS resolution issue because of a bad configuration, caching issues (I'm looking at you vodafone...) that even cause your TLS Certificats to be staled or even just replaced by your ISP.
If you did the proper investigations and don't get any troubles related to the upper metrics, then it's just Google screwing up so ^^
Some people have reported uninstalling and reinstalling Chrome helps, though right now I can't think of why this might be unless you spent a lot of time in chrome://flags messing around with things. If you have any VPN extensions, even if not in use, you should also try disabling them.
In my case I get Excellent connection when using Chromecast and controller but Game Might Stop (and often does) when using Chrome on a PC. I have a 400 mb/s connection. I get a "Great" result on the Project Streaming speed test. I beta tested Project Streaming last year with no issues. But now the PC chrome version is unplayable.