Despite i can understand what you mean, i would proceed with the memtest anyway (not because there Is a special coupon 
). There Is the possibility that they are not failing cuz bad, but maybe for some miss-setting on latency-frequency-voltage.
Download the last free release from the official website, flash an USB stick with Rufus, plug off everything from the Nas that Is not strictly necessary and boot from the USB. The test consists on 4 cycle, they will took some time to finish, but in my experience if something Is wrong they will be throw errors fast (but complete anyway at least the 4 cycle to be sure).
If no errors come up, i would start test the rest of the Hw adding 1 a variable at time, maybe try another os too. Instead if errors arise you have your culprit
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