It’s not actually a matter of overclocking, so much as NOT unknowingly underclocking with the default bios setup, which makes the nVidia interrupt capable of glitching Cubase. The I have since limited the i9 to 3700 mHz and it runs normally at just above room temperature, and only reaches a very tame 58C under 100% stress on all cores.
Have also found that if I limit Cubase to only the first 10 cores, then the very obtrusive nVidia interrupt will always occur on the unused 11th core, thereby not pulling the rug out from under one of the Cubase cores. The only combination that seems to do this is one that leaves core 10 (which is actually physical core 5) unused by Cubase, and the Cubase cores must be contiguous, no other combination of cores gets this result. So the Cubase suggested 14 cores is vulnerable to having nVidia intrude on a core in use by Cubase. Should also add the Cubase will not run if core 0 is not available to it. The upshot is that I can live with the video-editor friendly nVidia card. Can write up how to set this up if anybody wants.