Windows 10 VNC VMs intermittently hang on load and run

I've been using unraid for a little over a year now.  My goal has been to build a multi-headed VR gaming rig.  I had many trials and tribulations to get the system to a stable point with all the PCIE pass through and USB devices.  It was pretty stable for about 6 months.  Then I started getting intermittent hangs on my two main gaming VMs (one for Oculus, one for VIVE).  The hangs became more frequent until the system was unusable.  I ended up booting back to a regular Windows image without unraid and everything worked fine.  

A few weeks back, I made major HW changes.  Maybe my problem was HW related.  I changed my Xeon 22-core processor for a new i9 7900x.  I had to upgrade the motherboard from my Gigabyte x99 Aorus Designaire to a MSI x299 Xpower AC board.  With these two major upgrades done, I brought up the whole system again with a new install of Windows on a clean SSD.  Everything works fine in windows.  I booted back to unraid and had a look at my old VMs.  They all had problems.  The two main VMs would intermittently boot, then hang after 1-10 minutes of usage.  I had a number of relatively "pristine" VMs that I had only done a basic windows install with variations of BIOS and processor model.  These VMs also would hang, especially during loading.  I never got any error messages that I could sniff out.  The VM would just be shown as "paused" in the web interface.  If I force killed the VM, about 50% of the time it would no longer even launch, giving me some message about execution errors.  

So I decided that perhaps these VMs had become corrupted by having survived all the hangs in the past.  I deleted all VMs and built up 3 new ones.  I had no problems with these VMs during installation.  Then I decided to run some stress tests.  I ran combinations of 7zip, cinebench, and passmark.  All three VMs performed flawlessly for a good 24 hours of grueling CPU loads.  I brought the VMs down, increased disk space, and then launched them again to fix the disks in windows.  My first VM hung at BIOS.  I killed it and relaunched.  It hung during windows init.  I rebooted and tried the other VMs.  I was able to get hangs during BIOS and windows logo as well.  Also, I was able to fatally hang a VM by plugging/unplugging USB devices into a PCIE mapped USB card.  

What do I mean by "hang"?  The VM becomes unresponsive to input, doesn't update its output, which remains stuck.  Sometimes HTOP will show CPU utilization approaching 0 on all cores.  Sometimes, it will show all cores pegged at 100% (especially for launch hangs).  Sometimes, a single thread continues even though VM is unresponsive.  Sometimes, unraid will hang as well and requires a hard reset to recover.  

After a hang, sometimes I can relaunch the VM, and sometimes I get execution errors when I attempt.  Reboot resolves these issues.

So my conclusion is:

1) CPU virtualization is fine

2) Memory virtualization is fine (I suspected memory corruption as issue early on)

3) VM launch has serious problems

4) USB discovery via PCIE passthru has problems

And I have some big open questions:

1)  Why do my VMs get worse over time?

2)  What does a "VM Hang" actually involve? 

3)  Is there an architectural problem with KVM, virtio, or Intel's virtualization tech?

I'm going to give this a few more days to resolve, and then I'm wiping the whole thing and building up a couple of dual-boot windows build.  I may not be able to run 2 parallel sessions, but at least it will be stable.

--Brad