2011 iMac No longer Boots

[ArmouredBear]

My previously perfectly working 2011 iMac (fitted with 1 1.0TB SSD) suddenly crashed and restarted, after the Apple sign screen it just goes to a pale grey screen then restarts after a few minutes in an endless loop.
I have tried the following
Tried Internet Recovery - Same behaviour as above
Reset NVRAM
Boot in single user mode and check disk, fsck says disk is fine
Boot in Verbose Mode - No revealing Error Messages
Boot to a USB Boot Stick - It only sees the MainSSD
Boot in single-user mode and try to mount the USB Recovery Image, doesn't seem to be there. I can however navigate through the SSD file system no problem. The SSD is mounted.

Seems like either a hardware issue or something corrupted at the OS level but I don't know what I can do. I can't repair or reinstall the OS, can't run disk utility. Any ideas?

 

[Juicy Box]

It might be the GPU. I have seen something similar on a Mid 2011 27" iMac with a fried GPU. I fixed it doing the oven bake, and it runs great now.


But before you start looking into that, do you have an external bootable back up or another Mac to use?

Also, have you tried running Apple's HW Diagnistic Tool? In my experience, the HW Diagnostic Tool has a lot false negatives, but it might find something.