iMac will not start up and stuck in reboot loop

I have an early 2012 27" iMac which was running the last release of Mavericks OS.

Last week the screen flickered, then went black, then rebooted. It did this a few times over the next few days.

I did some research and decided to upgrade to the next OS (Yosemite 10.10.4). Ran the update, rebooted and then crashed half way through the white apple logo loading screen.

It did restart fine a few times and was working okay until yesterday when the screen went black and then it rebooted.

Now it is stuck a continuous reboot loop where I hear the chimes, see the white screen with he apple logo, the loading bar reaches about 1/4 to a 1/3 and then the screen flashes, goes, white, goes black, reboots - repeat.

I have tried:

1. Reset NVRAM (cmd + opt + p + r)

- white screen with apple logo, loading bar 1/3 full, logo disappears and garbage graphics line appears at top of screen, goes black, plain white, reboots

2. Reset SMC (power off, unplug for 15, plug in, wait 5, start up)

- white screen with apple logo, loading bar 1/3 full, logo disappears and garbage graphics line appears at top of screen, goes black, plain white, reboots

3. Safe Mode

- does not boot into safe mode

4. Boot to Recovery HD

- does not respond to CMD+R

- responds to OPT and I get:

-- hardrive (white apple logo screen does nothing)

-- recovery 10.10.4 (boots as above with the reboot loop)

-- install dvd (white apple screen does nothing)

Anyone have any more ideas about how to access a useful menu so I can run some diagnostics and recovery?

Any thoughts on what hardware is more likely to be the issue as this started happening BEFORE I ran any software OS update?

I've repaired many apple laptop and macs by swapping out hardware - so I don't mind trying this option.

Thanks