recurring hard crash in games - specs inside

* Intel i5 2500k * Asus P8Z77-V motherboard * Sapphire Radeon 7870 gpu * Crucial 8gb(4x2) DDR3-1600 ram * Crucial 128gb m4 SSD * Seagate Barracuta 500gb HDD * Corsair HX750 PSU

I built this computer about a year ago and have had this problem ever since. Haven't been able to narrow down the problem and I'd like to before I throw any more money at it than i already have.

The computer crashes only when playing games. It will crash to a black screen(I've never seen a blue screen on this PC) and I have to hard reset it. Sometimes this happens after 10 minutes, sometimes 2+ hours. CPU and GPU temperatures look fine, in fact I've run GPU stress tests and mined bitcoins for 24+ hours without issue.

I've tried swapping out the following components:

* replaced the GPU with an old nvidia 9800 I had that never gave me problems.

* replaced motherboard (originally asrock z68 extreme3gen3)

* replaced PSU (originally corsair 650w)

* replaced ram (originally kingston DDR3-1600 8{4x2})

* I originally had windows 7 installed and upgraded to windows 8

My next thought is to unplug the SSD and install windows on the HDD. A few issues I've run into with that: * I don't have the win8 setup files or ISO(originally installed from USB stick, since formatted). I do have my win8 key.

* I don't have a windows 7 install dvd or key. I was using an OEM copy from a friend and no longer have access to it.

* I bought a win8 upgrade when it was $15 shortly after release, will this cause me problems doing a clean install with this key?

If there is any more information that would be helpful i'll be glad to provide. I'd like to narrow this down before spending any more money replacing components if possible.