MacBook pro 820-3787-A

the macbook start normal untel i put the password stay 1 min and restart with black screen
if i turn it off and start it again the mac work normaly and after 1 min repeat the problem.
i change the pasta under the heatsink but nothing change.
i dont know from where can i start to measure in this setuation??

if the macbook stay at the first screen that ask about password , stay work without restart
i change the ssd but the same problem

Probably GPU issue problem.
Install gfxcardstatus and switch to iGPU and check if stays on more time.
Also try starting in safe mode.
ASD can also report some errors; OS version recommended to test discrete GPU.

If problem persists under Intel graphics, then you can have FDMF6808 issue...