Not sure if you have solved this , I noticed you mentioned that you had no monitor connected, I had the exact same issue except i had a monitor connected, the rig would run fine until I turned the monitor off and then it would freeze, as long as the monitor was on, it would run fine.
at first I thought it was ram, I had cheap ram that i got off facebook market place, 1 of the sticks failed a memtest and i was running usb stick that had always been slow from day 1, bought 2x 8gb of ddr4 and replaced the usb with a samsung ssd, made a huge difference in boot time and updates and installing miners, was slow as all hell before, but sure enough when i got up in the morning it had frozen again, going by the time on the screen it was not long after i turned the screen off.