Hardware or driver? As in "not Windows or applications" you mean there I assume? I would have assumed so too and more likely hardware given the first machine with identical hardware did not exhibit the issue. Just as I was typing it I remembered my issue with the other machine.

Machine is well within warranty. Only about 3 months old.

I did get a new power adapter with the new machine and I'm 99% sure I kept it (I had both of them side by side for a couple of weeks).

I've just installed latest Intel video drivers (not from Dell).

It can take hours or days for the problem to (re)occur