I have a MSI Radeon 7870 2GB Twin Frozr with an I5-3000(can't remember the number exactly).
My problem is that while gaming for an extended period of time (mainly playing LOL, but also during CS.GO) i can hear a loud fan-like noise coming from the case, the screen goes black and I'm forced to shut the computer down manually and reboot.
I have tried all kind of solutions: changing the power outlet, opening the case to improve ventilation, underclocking the GPU.
I even tried reproducing the situation by running a Furmark CPU+GPU for about 15 minutes and it didn't happen. It only seems to occur during longer periods of gaming / use.
Do you know what could be the problem? I think the GPU has something to do with it, but can't pinpoint what.
Sounds like you might be having temperature issues, use a program like SpeedFan and measure your temperatures both of gpu and cpu while playing.
Running Furmark for 15 minutes might not be enough to get to that high temperatures as playing for some hours. Do your system breath well? I mean how many fans do you have and how good is the air flow? Also, is your pc in a place that can actually breath?