Hi, I don't know if I have the exact same problem but it looks like this. Every time I turn on my iMac, it shuts down during the boot process and restarts. Sometimes up to 10 times in a row. When the login screen is reached and I log in, the iMac also always shuts down within a few seconds to a few minutes. However, if I use this iMac in target display mode (cmd-F2) with another iMac, it no longer shuts down and I can use the iMac all day. If I log in to the iMac and then do nothing with it (no mouse movement and no keyboard), the iMac also stays on. Does anyone know what kind of defect is causing this? And why does the iMac never shut down (reboots randomly) when i use it in target display mode??

FYI the iMac on the left is a newer one with no issues. I usually (after a lot of spontaneous reboots) use the right iMac in target display mode as a second monitor for the left iMac. At 5.25min i perform a PRAM reset twice (without positive result). All reboots in this video recording occur spontaneously.

I recorded my issue. You can see it on https://www.youtube.com/watch?v=eF6uiHwC...