its not a motherboard issue,it just need to replace with orignal or good copy and more important keep the same orignal charging block,i have come cross many time with this issue mostly it happen to me when i replaced the charging block with after market ones,then i have to source good copy or taken out from other iphone and problem was solved permanently,phone was restarting after evey 4 mints.when i used orignal part restarting problem was gone competely
this help