I replaced the power supply with a refurbished apple oem supply and it is STILL rebooting. New lithium PRAM battery, new known good RAM, new SSD, clean install of El Capitan, zapped the PRAM and reset the smc all to no avail. STILL REBOOTS even after shut down.
I too, took advantage of replacing the 3V #2032 PRAM lithium battery. Doesn’t make sense not to, since it’s completely torn down.