I have had what sound like a very similar problem - running Parallels 15, Windows 8.1 on MacOs Catalina 10.15.7 - where whole device just randomly freezes for many minutes and is difficult to recover as described by many here. I think I may have found a resolution for my set up by following the solution in this 2015 Parallels Knowledge Base article:
https://kb.parallels.com/119992 "Windows 8.1 hangs with the black screen for several minutes"
You have to uncheck "App Readiness" under the Services tab in Msconfig due to a Windows Update misconfiguration. My system had "App Readiness" checked and so far has been stable since unchecking this.
Whether this is relevant to those running later versions of windows I have no idea, but may be relevant?
https://kb.parallels.com/119992 "Windows 8.1 hangs with the black screen for several minutes"
You have to uncheck "App Readiness" under the Services tab in Msconfig due to a Windows Update misconfiguration. My system had "App Readiness" checked and so far has been stable since unchecking this.
Whether this is relevant to those running later versions of windows I have no idea, but may be relevant?