I hesitate to post this since it doesn't make much sense, but I'll just describe what happened and others can do what they want with the information.
System:
AMD 3900X
32GB system RAM
RTX 3080 10GB
Settings:
DX11
1920x1200@60hz
Default high
Frame rate cap 60FPS
DLSS: Balanced
I was having pretty much the problem described here. Sometimes, the game would maintain a perfectly smooth 60, but then inevitably it would start to stutter and FPS would drop to a variable 30-50. I tried everything to fix this including switching to Vulkan, setting all graphical options to low, wiping out the user profile, reinstalling the Nvidia drivers, etc. Nothing made any difference. The GPU would report 100% usage, but GPU temp was hovering around 50C, so clearly it wasn't working very hard.
Then a couple of days ago after a full settings wipe, I tried switching DLSS to "Performance". GPU usage immediately dropped to around 50% (expected with a 60FPS cap), and suddenly a smooth and consistent 60. I even switched DLSS back to "Balanced", and it maintained a consistent 60.
So that's where I am now. Game is running as it should, but I'm not entirely clear what I did to fix it. All I can say is the last thing I did was the DLSS switch.