For a fast answer see last paragraph.
I had similar issues with my home lab server:
2 Socket Intel Xeon CPU E5645 - total 12 cores 24 threads
64GB ECC ram (4x16GB)
SuperMicro X8DTI-F
2x WD RED 1TB
TOSHIBA 4GB N300
Samsung SSD
Qualcomm Atheros AR93xx Wireless Network Adapter
Marvell 88SE9120 SATA Controller
VIA VL805 USB 3.0 Host Controller
I normally run 4-7 VMs on this machine. The main ones are:
pfSense router/firewall/access point (with pci pass-through)
FreeNAS with mirroring (with onboard Intel 82801JI SATA Controller pass through - WD Reds)
Virtualmin (CentOS) web server
Windows multipurpose vm
After trying many things I narrowed it down to power issues. It was either an unstable motherboard (didn't seem likely because of sudden onset after years of it being rock stable) or a bad PSU. I changed the PSU to a Corsair RM850x 850W. The issues went away for many months. I didn't have any issues with high loads or starting and stopping VMs after that. The issues came back all of a sudden one day executing a CPU demanding task. After that I could not start all my VMs. Starting one or two where ok, but If I tried to do a normal boot with only my main 4 VMs I would get an instant reboot. I could not bring myself to blame the recently bought PSU.
You see, what I failed to mention so far is that I am a moron
(slapping myself).
Also, that all this equipment was being powered by a slightly undersized and maybe failing EATON 5E 850VA UPS (I know - I was just being cheap). Changing the Power supply to the RM850x may have masked the UPS issue in the first place. The Power Supply I had before the RM850x still works on other computers without issues.
Removing the UPS from the equation solved everything. I hope the exposure of my stupidity makes someone else act smarter and save some time