Than it seems that you suffer from a different error (or the one reportet by Kurtis Raader)...
It seems I was a little hasty in my workaround testing. Not sure what happened but my original testing with iommu=soft didn't produce anything positive. Based on your comments I went back and tried again. Lo and behold (cross my fingers) since adding iommu=soft I have been running for 3 days without an error.
Perhaps I made a typo in editing grub.
Performance impact with this switch hasn't been too bad. Maybe 3-5% in increased runtime for my jobs. Still it would be nice to get a fix and I still worry if some of my O/S has been damaged whilst installing the O/S with buggy disk writes.