This system suffers from intermittent restarts that occur for no apparent reason. Examine the figure (click to enlarge).

Question: What is the most likely reason that the system is intermittently restarting?

Bonus: How can we fix the problem?

(See the answer and video response below.)

Answer

This is an HTPC that I built several years back. When I first built it there were indeed intermittent restarts. The cause was a wire that was brushing against the back of the case resulting in a short which happened randomly and ended up causing the system to restart.

The solution was better cable management. By tying up all the cables – as shown in the figure below – and making sure that no wires or connectors touch any metal of the case or any circuit boards. After doing this, the system ran fine for a test period of 48 hours. Then, I added some rubber standoffs under the motherboard and the power supply and made sure everything was well insulated. The system ran strong as an HTPC and media server for 5 years. I don’t have much of a need for an HTPC anymore so I’m recycling it into a secondary Windows server.

See the video solution for the actual problem in this case and the followup video for more about other potential culprits.

Video Solution

Answer at 0:36

Followup:

So, the question asked for “most likely” making the question more of a hypothetical one. Now, if we are simply talking about the most likely cause for any intermittent restarts my troubleshooting list would be as follows:

1. Overheating CPU due to disconnected CPU fan or disconnected case fan (or possibly overclocking or failed thermal compound though I put those farther down the list usually).
2. PSU is underpowered
3. PSU is faulty
4. Short somewhere in the system
5. Capacitor issue or other motherboard damage

And that’s just on the hardware side of things (and not a finite list mind you). Restarts can happen for lots of reasons on the software side too: malware, OS issues, driver problem, and so on. Troubleshoot!

Followup Video