Having already rolled back the BIOS for testing and seen some measured degree of improvement in stability, I've also been testing the 390 series of nvidia drivers as many have been complaining of stuttering (beyond that caused by the power monitoring bug which is another thing). I'd mostly been using Battlefield 1 for testing. Well, BF1 got updated which kinda screwed up my test bench (you shouldn't go changing things when you're testing!) but after the update I observed stuttering which would better be described as freezing - if I entered the menus in game, the whole screen would freeze for 1-2 seconds before the menus appeared or disappeared.
As an extension of my testing of the interaction of the new drivers and the spectre fixes, I removed KB4056892 and boom, freezing and stuttering is gone.
It should be noted that others who do have this patch installed, do not experience the stuttering/freezing that I recorded. I am using a 5820K and it appears that the issue may be processor/generation-specific (as in, this is happening to haswell but not kaby lake)
This whole spectre 'fix' is clearly a mess. Let me be clear here, I am NOT advocating that people run their systems without security patches. This was done as part of a testing protocol and SHOULD NOT BE CONSIDERED A SOLUTION. I'm just keeping you in the loop as to the results of my testing. I have escalated this to nvidia and I would expect that they will interact with microsoft to fix the patch. I have also logged a case with microsoft directly. I'll let you know as things pan out.