Dell Precision T5820 becoming unresponsive

I have had a few episodes of this occurring over time, sometimes several months apart. The computer power light is on but the hard drive activity light does not flash. The computer won't respond and requires a hard shutdown which fixes the problem. I had always assumed it was a Windows related issue. I use the computer to run CCTV software so it is going 24/7. 

Recently I moved to Proxmox and am now running Windows in a VM. The weird this is that after having had this new setup running without an issue for several weeks, I moved it into my garage. Three hours later, the issue happended again. This rules out a software issue it seems. 

From what I have read, hardware issues could include power supply, a failing hard drive, or overheating. I did have the rear only a few inches from a wall but I feel that I would have heard the fans if it was overheating because they are very loud when they full at full speed. I recently replaced the motherboard, so it seems unlikely to be that given that it did this with the old one. 

It was plugged into a power board which I have now removed. Hard drives report good health on SMART. 

Has anyone had a similar issue? If so what was the cause?