Question I am having some problems with my pc after swapping out some parts

Feb 14, 2019
I swapped out my motherboard, cpu, ram and got a new harddrive. I got a few problems. First of all after I Installed windows I got problem booting my pc. It turned on and off 3 times until I came to a recovery bluescreen. I thought it was something wrong with the harddrive so I sent my pc to a company for repairing it. When I got it back I was able to turn on my pc. They said it was corrupt OS, but after like 5 min with the pc on it turned it self off. And the same thing happened again. It turned on and off 3 times before I got into a recovery bluescreen. Then I tried plugging one of my screens out since I had 2 plugged in and then it turned on again and I was able to play for about an hour before my restarted by itself and now I cant play a game at all and sometimes I have problems booting my pc. Like it turn on and off 3 times and sometimes I get in and sometimes I get a bluescreen. And I am also having problems reactivating windows 10. I have a windows 8.1 product key if its to any help and I upgraded to windows 10 when you could do it for free.
My specs:
Motherboard - ASUS ROG Strix B360-F Gaming, S-1151
CPU - Intel Core i7-8700 Prosessor
RAM - HyperX Fury DDR4 2666MHz 16GB
SSD - Kingston A1000 240GB M.2 SSD
GPU - Geforce gtx 750 (Vram - 984mb)
PSU - Cooler Master B500 V2 500W

I think the problem is that the gpu is too old for rest of my components or the gpu is broken or aint getting enough power. It might also be because I dont have activated windows 10
Any suggestions?
PS if my english is bad in this thread its because it isnt my main language. So sorry :p