We have a few Nokia Lumia 610's and they keep restarting The phone is left on a desk then it plays the start up music and displays the enter SIM Pin page. this happens once to twice a day on all the phones we have. They are all been updated to the latest version available 7.10 and it still has this issue. The battery is charged and this also happens if the phone is connected to the charger.
So, i bought a lumia 640 yesterday and I had the same problem so i did some testing and i think i found the solution.
(sorry for my english) On my testing i found that:
if i disable the location services and the options that are found on the "manage" in the wi-fi setting
and when the phone is locked
and if you moved (like from your kitchen to your garden) for some reason the phone restarts!!!
So i activated all this settings again and the problem is resolved....
Befor this i tried upgrading to Windows 10 on my phone and it did not solve the problem. So this seams to me to be the solution!
Can you check this settings on your phone?
PS: it is very strange that MS does not fix this thing! I did some search on the web and found that this is a recurrent problem on any Windows phone device! MS fix this please!!!!!