Same problem for me, happend in September. I didn't find any workaround, and don't want to bother with board exchange so a buy a new phone.
I tried to open it, disconnect the battery bit it do not want to work when plugged in without the battery connected. When it charger it is working but extra slow, and when disconnected it reboots every 3 to 10 minutes. I tried hard reset but it does not change anything. If someone knows how to fix it without motherboard swap I'm interested!