Imac 27' from mid 2011 reboot issue

[Pilou57]

Hi. I have an iMac 27' from 2011 and it keeps on rebooting randomly. I can figure out why this is happening. I thought it was the battery on the motherboard, so I changed it, but is still happening. Any ideas? Also what is a bit weird, is that I did reset it and reinstalled the IOS, and through out the process the iMac was perfectly fine. But once it was over, I unplugged it and relocated it to a new desk and ever since it keeps on rebooting again. It is like a reboot loop at the start. Sometimes I get to the point of entering my password, sometimes it reboots even before that...