My dell is having big problems!

My laptops motherboard (CN-0HNR2M) is having problems.

When I push the switch button, it starts working, but after 30 seconds it shuts down automatically.

I then dissembled my system and applied new thermo paste on CPU, as well as new thermo pads on GPU, North and South bridge chipset. but nothing has changed!!

I disconnected the yellow cable of CPU fan so it works at the highest speed, but GPU is still HOT, and the laptop shuts down like emergency. I think there is some kind of problem of any little resistor, or capacitor as the GPU appears to be getting to high a voltage and that's why it gets HOT (between 70-85 celsius )

So what can i to do ?

thanks a lot