Loading, Restarts, Repeats (iMac 27 2011 i5)

[LAHegarty]

Hi, looking for things to try

Turns on loading bar displays and loads all the way then the screen grays out before loading OS login screen, stays on for about 30 seconds then restarts and repeats.

To no avail, I've tried the following:
- PRAM reset
- SMC reset
- Tried different RAM
- Installing the OS on another working iMac and swapping the drives

What else can I try? Has anyone else had this happen and what have they done?

Thanks, Luke.

 

[casperes1996]

When you boot up the Mac, hold down cmd+V to get a verbose boot up and see if you get any indications as to what the issue is

 

[LAHegarty]

Thanks casperes1996

Made a video:

 

[casperes1996]

Thanks casperes1996

Made a video:

Nothing seems to be the issue in the boot process itself - When booted in verbose mode, you don't get a kernel panic message when it restarts?

Do you get any messages about hardware errors, if you boot into Apple Hardware Diagnostics? (Can't remember the key(s) to hold on boot. I believe it's cmd+d but try and Google that one)

 

[Fishrrman]

What version of the OS are you using?
Have you tried a safe boot? (boot with shift key held down)

 

[LAHegarty]

I've tried 10.7 and 10.13.
Holding shift does the same thing, a grey screen at the end then restarts.

 

[LAHegarty]

You hold down the 'd' key after chime for the hardware test, it's running the test now...

 

[casperes1996]

You hold down the 'd' key after chime for the hardware test, it's running the test now...

Yeah - that's it. Hope this shines some light on the potential issue