Yeah, after retweaking my undervolt settings with the new drivers, my 4090 has been just fine. Most of the instability I found was in RT/PT games with DLSS enabled using the new Transformer "K" model. I am left to assume all the new, extra workload on the tensor cores exhibits enough additional load to make my prior "stable" undervolt actually not so stable as I thought.
I still have a nagging instability in Space Engineers, even while running 100% stock GPU settings (volts, speeds, etc.) I think somehow it's related to VRAM and I"m wondering if a recent game update may actually be the culprit there.