MacBook Pro freezing and starts again after many restarts

Hi, I have a macbook pro with this configuration:

Model Name: MacBook Pro

Model Identifier: MacBookPro8,2

Processor Name: Intel Core i7

Processor Speed: 2,5 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 8 MB

Memory: 16 GB (2x8)

HDD: SSD 120GB

In the past weeks something weird is happening and I'll post some pictures to understand exactly what my problem Is.

When I'm trying to open my MBP (I'm not talking about waking it up from sleep) the display looks something like this:

of corse I've tried to reinstall the OS X but during the install I've got this (eventually I've managed to reinstall the os but the problem was not solved)

One thing that I've noticed is that when I unplug my MBP from the power source in that moment it freezes and There is nothing else to do than to try to restart it again and again and to hope that eventually it will start up. Eventually it opens and is working perfectly, without any problems. So.. .I was thinking that maybe is a graphic card problem so when the laptop finally opened I've tried to switch graphic cards and guess what? FREEZ!

I've uploaded to youtube 3 videos to see exactly how my macbook is acting when I'm trying to open it (video1). How is freezing when I'm trying to switch the graphic card (video2) and How it looks when my laptop is finally able to boot "correctly" (video3)

Any ideas how to fix this? If this is a graphic card issue is there anyway to fix it or to change it?

https://www.youtube.com/playlist?list=PL6ReoC1oRDtnV-qM9VBwEgJdcEBlYy5LN