What you can do quite easily, is run a program which will read the temperatures and voltages, for example SIW (System Information for Windows)(1)
That'll give you some idea as to what's going on under the hood.
The M1000, being a group-regulated design, wasn't the most stable of power supplies. I don't say it's bad, just imperfect.
The +12 volt rails should never drop under 11.4 or go over 12.6 volts.
5 volt rail should always be between 4.75 and 5.25 volts.
3.3 volt rail should be between 3.13 and 3.46 volts.
Power noise and ripple can't be measured without a oscilloscope.
If you see nothing out of spec, please try temporarily removing all the extenders and run a GPU plugged directly into the mobo.
If the rig hangs again, change that card to another.
Report whether or not you can get this thing stable.
Should the rig stabilize, add as many cards as you can without using the extenders. If everything still works fine, re-apply the extenders.
It's a lot of work but it's bloody damned hard to troubleshoot a (suspected) hardware issue remotely.
Best of luck!
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