Is this restarting happening randomly or is it happening with a specific application? If so then try removing the application otherwise what you could try is to do a soft reset *#7780# with default code 12345. This will reset certain settings on your phone but will not erase the device and see how it goes. If it still keeps rebooting then your need to try a hard reset *#7370# again with default code 12345. Please be aware that this reset will erase all data on your phone so make sure you back up your device first with Nokia suite.
Also using Nokia suite you could try to re-install your firmware by going into tools and then software update hit refresh and the re-install,
If after all this your device still keeps restarting then you may need to visit your local care point! Also try another sim card as sometimes a faulty sim can cause restarting issues.