Well, the important point being that I’ve rolled back the driver to the last previous version (whether it was .06 or .22, I wasn’t paying attention, but it began with 452) that was available to download on Nvidia’s downloads list underneath the latest September one. Reverting to the previous driver has cured the major stuttering issue. Others have confirmed this corrected the problem for them, too.
Three days ago, I had framerate drops again!
The issue was the geforce experience application..
Solution: turn off auto-optimization in the application! → Disable Game Optimizations
Hoping this helps someone 