Hi @ggwilson ,
There may be a problem with the PSU drifting outside the specified parameters when under load, as the Standby voltage (no load) seems within tolerance. Also it appears to affect the -ve voltage supplies more then the +ve voltage supplies.
If the -12V DC supply is reading -14V DC what is the +12V DC supply showing?
Usually the tolerance for a PSU is +/- 5% for the various voltages being supplied. Some of the other voltages that you have shown also don't seem to adhere to this e.g. the +/- 5V DC, especially the -5V DC as they both should only be 5.25V DC max. (although the +5V Standby voltage seems OK - no load?).
For the -12V DC supply only it is +/-10%, which would place it between -10.8V DC to -13.2V DC and not -14.2V DC as you apparently have.
Check what is written on the PSU itself to see what the specs are for it.
A PSU is a voltage regulator which means that it supplies a consistent, constant voltage output that should not vary outside the tolerances shown. What does vary is the current being supplied by the PSU, that is required to suit the demands of the PC at any given time.
I personally think that the -12V DC supply is too high and perhaps would be placing the components under a bit of stress by making them work too close to the top end of their max. voltage rating, e.g. some 12V components might only have a 16V max. voltage rating. At the lower voltages even 1V can make a difference sometimes.
Also use this PSU calculator to work out roughly what the PSU total output wattage should be for the desktop. The manufacturer may have installed the minimum that will work but does not really cater for any expansion that may be done.
You didn’t say but have you checked in Event Viewer to see if there are any noticeable Critical, Error or Warning events that show up or stand out about the times that the restarts occur?
If you have Win 10 installed, right click on the Windows Start button (left side of the Taskbar) and click on the Event Viewer link in the box that appears.
If there are any events that stand out, doubleclick on them to get more info and then search online using the Event ID number and the Source to find out more about what happened to cause the event