It has been known that this phone has problems with its software and its operating system. The best thing to do is to perform a factory reset. You can do this by holding Volume UP + Menu + Power button until you will see a menu with a text. Navigate using volume buttons and select Wipe Data/ Factory reset. Press Menu as it's the OK button, then restart the phone and wait for it to boot properly.
This should fix all the problems that it may have. Also, please notice that the reboots may be caused by different applications.