OK guys, a bit of background!
Had the RROD last year and sent it off to an online store to fix... didnt take too long before the RROD came back. So in a 'NO LOSE' situation I thought I would go about fixing it myself.
The company had done an X-Clamp fix but I decided that I would start from scratch. I cleaned the CPU and GPU with Arctic clean and put on some fresh Arctic silver. Only a little bit as described in the tutorials section here.
I added a front header for a fan and bridged the 12V connection as shown
HERE Everything looks good and there is a good conenction. When plugging in the fan, it works faultlessly. It is angled as per the tutorial. The fan I installed was a Coolmaster Ball 12v - 70cm.
I also made the cooling shroud as described
HEREI also put some GUIDES so the airflow of the fan is directed under the GPU Heatsink.
Anyway, I had to let it overheat for 2 minutes EXACTLY.. as described on another RROD fix. and the XBOX 360 boots up EVERYTIME. Without fault.
When I play games though, I only get a couple of minutes in and the screen freezes. This even happens on a loading screen?

When I go to the back of the XBOX the air coming out is quite cool, so there seems to be good airflow.
I have also slightly loosened the screws on the GPU and CPU Heatsinks. And also put INSULATION TAPE on the metal base, where the screws from the clamps might touch.
SO I am at a loose end. Tried it with the EXTRA FAN plugged in, and taken out and it still seems to happen. Getting NO GLITCHES, No RROD or anything?

Help appreciated coz there is NO other mod I can do?

Simulcra