Mobile restarting is caused by many reasons; fake charge, or faulty sim...
Let's try solving the first reason by changing your charging usb or the power source if it's not established well while charging (If mobile randomly charge and discharge while charger pluged in) because fake charges causes mobile lag and restart to get the real status of charge. Try this step after completely discharging your battery (0%).
The second step could be solved by changing your sim with another one that fit it's place in the socket without being moving inside it because as you know, the mobile restarts during inserting new sim or removing old one and if your sim is moving inside the socket.. It may cause restart.
I don't know other solutions, but I hope it be fixed for you.
