GPU Freeze/Crash

Hi,

I have an Asus UX32VD with both an Intel 4000HD integrated card and a Nvidia GT620M GPU. Im running windows 8 (but problem happened with windoes 7 as well).

The computer runs fine (with the exception of the problem of not always shutting down which I belive is fairly common in windows 8) except when running any graphics intensive program (normally a game). After a few minutes the whole system freezes up. The screen output is "garbbled" (a mixture of green and a static image of what was there before the freeze) and the sound buzzes.

I can reproduce this most times by checking the windows experience index. Which casues the same sort of crash after a few seconds.

If I disable the Nvidia GPU it dosnt happen.

If I disable the Intel chip and try to run just the Nvidia GPU the computer dosn't recognise it (even though it is still present in device manager).

I've checked the Temp using Asus GPU tweek and the problem can happen anywhere from 60 dgrees to 75 degrees, so I'm not sure its overheating.

I've read there can be problems with Nvidia optimus if you have two GPU's but I'm not particulaly technical so im not sure how to test this. I've tried opening up the Nvidia control pannel and manualy allocating a game (GW2) the GT620M card and while that works it still freezes after a few minutes of playing.

Can anyone help?