I have a same problem. It restarts radnomly, sometime few times a day, sometime it works fine for a few days. I reported the issue to the seller and they reportedly contacted supplier. This is a quote of their message : "Please help us open the phone back , and the shrapnel is in poor contact. After I asked if I should open the phone!? they said: "Please ask the professional person help you"
Does this make any sense?
No, it doesn't. If the phone is still under warranty, seller have to repair/replace/refund it. You have to open nothing. Feel free to screenshot and forward my message.