Be sure that all important data is backed up to at least 2 x to locations away from the system. Verify that the data is recoverable and readable.
= = = =
More testing:
Continue working with Task Manager and Resource Monitor.
Try Process Explorer (Microsoft, free).
https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer
Boot the system and close all running apps, utilities, and so forth.
Then use one of the tools to simply watch the system for awhile, Hopefully the speed changes will stop and the system will remain stable.
Next, keeping the tool window open to watch, begin launching apps. One app at a time and wait/watch a few minutes between each launch.
The objective is to discover what app (or combination of apps) causes the speed to start cycling up and down again. Repeat as necessary using different tools.
Do not run any bench mark software.
If the cycling does not occur then repeat the process while running one of the benchmark utilities. Determine if the cycling occurs with one or some combination of benchmark utillities and apps.
Objective is to discover some set of controllable changes that you can make to cause the system to cycle up and down. And then be able to stop the cycling by reversing the change.
Take your time, be methodical, keep notes. Take screenshots for reference purposes.
= = = = Hardware issue ? = = = =
The next step will be to swap in another PSU. Do you have another PSU available?
Be sure to use only the cables that come with the swapped in PSU.