It could be battery related
Might even be that the battery is moving inside the phone and temporarily removing power. Take the back off the phone and see if you can duplicate the problem by trying to move the battery in all directions.
Perhaps the back is not pressing down enough on the battery to keep it in place? You could try wedging a piece of folded up paper between the battery and back to see if that makes a difference.
These are not solutions BTW; just trying to narrow down what the problem might be.
You could take it in to a carrier store to see if you can try a different battery.
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