Pc Reboots While Playing Game ( Mostly on CS:GO)

[BazzerBiller]

Hello, almost 2 months ago I have upgraded my pc with new PSU and GPU because of my dead GPU(Gtx 760). Ever since then, pc randomly reboot while playing games. I say randomly because there is no specific game, sign or time. FIrstly, I approach on lowering temps. Since I dont have enough cooling in my computer I have open one side and did UV/Fan curve settings for my GPU. Before that, CPU temps were 80-85 and GPU temps 73-76. After opening one side of the case and doing UV results were satisfaying enough to stop this issue( for a while...) GPU temp went 65 max., and CPU temp were 65-73. I was playing Pubg, Cod Warzone and Apex. Games like demanding and using my GPU fan curve like crazy. Then, I started to play CS:GO ( in last 2 weeks). The problem occured again? Like W? my CPU temp max 65-66 and GPU was 53-55 :D. Reboots highly increased and resulting in a pretty random times. It happened on workshop maps, mm matches or menu? I assume the previous attempts were just a short term solutions and the problem getting bigger now. But I dont know whats the problem and I cant send my computer to retail store because of my school. But, I just want to solve this problem.

Pc:

i7 3770(non k) 3.4ghz - turbo boost on ( its a 7 years old CPU )

16 gb DDR3 Ram 1600 mhz - Dual Channel

Rx580 8 gb- I have tried with overcloking, undervolting and stock settings; rebooted all the settings :(

1 TB HDD- kinda old ( 3-4 years )

HighPower 850W +80 Bronze Power Supply

B75M-A Motherboard

 

I am really open for any ideas.