Black screen, no fan, or rarely screen flickers

Hi,

I have a Sony Vaio VGN-NR21Z (model PCG-7121M). It was working well for a few years (2009 to 2013).

Then after what may be a little overheating, the screen began to show random flickers (like horizontal lines and sometimes unstable picture). A few days later, the computer wouldn't even boot.

I was about to junk it, and tried again today. The problem is the same : only the power LED lights on. At first, no noise can be heard, no fan noise, no beep.

What I noticed today is that after a while of being powered on (about 5 minutes), the computer's fan starts up, because the CPU is getting hot (I guess).

And what surprised me today is that, while it was hot, I restarted the computer and could ONCE see something appearing on the screen : the VAIO logo with strange squares, and maybe the BIOS lines saying that there is no hard disk (I suppose this was the message, because I removed it, but I'm not sure, of course).

I couldn't reproduce this behavior, unfortunately, and I'm now returned to the black screen.

Given these symptoms, can you help me identify what the problem is? The fact is, this computer has a NVIDIA GT8400M which is known to have the NVIDIA defect. I would like to be sure that this is a NVIDIA GPU failure and not something else.

Thanks!

Update : I opened the notebook and used a hair dryer on the GPU to heat it. To my surprise, the VAIO logo appeared without any distortion, and then the message is «No operating system found» (which is normal because I removed the hard disk). Unfortunalety, when the GPU is cold, then either the screen has flickers, either there is the black screen.