My iMac will not load OS

My iMac will not load any os, I can hold alt and select any drive both internal and external, i can run new installation and format internal an external drives but once installation is completed and i try to boot os the progress bar goes across and it goes to blank grey/white screen.

i tried the following,

1) Switched hard drives

2)boot from external lion disk

3) changed ram

i don't think its the graphics card or sata cable because i was able to load and complete the OS X El Capitan installation.

-- I've tried booting from an external HD (not the original drive, hooked via firewire and then by USB), a usb thumb drive (once with a Snow Leopard boot, then once with Lion) and of course the install disk. Displays the same behavior for all .

-- Attempted to boot in safe mode (CMD Shift):

-- --Displays apple screen with the progress bar

-- --fills the bar about 30%

-- --bar disappears and is replaced with spinning gear

-- --gear freezes, no boot

-- pulled and re-set ram in slots (not original RAM, but replaced w Kingston RAM when I got the machine in early 2008, and there have been no probs until now). Tried switching slots, tried each ram card alone in both slots.

-- PRAM Reset: CMD + option + P + R

-- SMC Reset: Unplugged for 15 seconds, plug and wait for 5 seconds, press power

It will go into single user mode, so i imagine the solution has to come from things I can do at that command line.

Please don't make me go to genius bar!

After doing some indent research i found that this model has a 5 year recall/warranty on the GPU see video for details https://www.youtube.com/watch?v=QgQAtbum...

And this is what i used to repair. I now need to prevent my god core from hitting 200deg Cel or keeping as cool as possible

https://www.youtube.com/watch?v=9vPM41Zm...

I now need to prevent my GPU core from hitting 200deg Cel or keeping as cool as possible any ideas ?