I had similar problem, after an update i done it started restarting, wasnt that bad it was just once or twice it happened and maybe a freeze here an there.
I then done a hard reset/full clean of the device. Basically just what the others here have mentioned.
Flash the eMMC first with the vanilla firmware (im not sure if there are any new 1s available but do check and use which ever latest 1 you can for the emmc) then flash the device with the latest firmware.
Use the flasher tool to do both of these things and hopefully it should have resolved the issue.
Or alternatively if you are unsure how to do this and don't wish to go down this route then you can always take it back to the place you got it from and tell them to fix it or give you a replacement. If they ask you to wait a couple days or something then just moan at them, get a refund and go somewhere else get yourself a new 1 there and then