I have a solve, or at least can point to the cause of this issue. It's something to do with this specific NVMe drive. Not sure if it is the lack of DRAM or the new controller on the 980, but whatever it is, that is what is causing this 30 min period of reported overheating.
I just switched out NVME drives (in this case to the 980 Pro) and the issue is gone. No other change in configuration at all. Even using the same M2 slot on the Motherboard. Been running solidly like this over 48 hours now.
I'm not sure if this is something that Unraid can fix, or if it will get better with newer releases that deal with this new style of NVMe drive more correctly.
One parting thought on this. I don't believe the 980 actually ever got to 84 degrees or that it overheated at all. I think it was just misreporting. I made sure to be there a couple of times when it was reporting the overheating and the drive felt cool to the touch. So I don't think over-time, this problem will cause any issues to the physical drive, but since this was a mission critical drive for me, I didn't like the issue and wanted to be sure to get onto something which works as it should.