I've got the same problem, always restarting when playing a game. Please sort it out ASAP as I've also bought my kid's this phone for Xmas and they will be disappointed!
I upgraded to J3 2017 in late October this year. All fine including apps. November software upgrade completed and then the problem arose. I contacted Samsung - not helpful. No idea if or when fix is to be issued. My provider were more helpful. New phone. Great until that upgraded itself. Back to square one. If Samsung monitor this website they should be aware that i am tempted to contact nation media for help. I am sure they will do something.