Computer Crashes/Restarts

At first I was looking around for answer.. But now I'm going to ask it.
OK.. The computer fans and interior are clean. I put thermal paste on CPU. And that did not work. The problem I am having is randomly why im doing something, the computer will crash in one of 3 ways. I'll here it make the sound as if it shut down, but i can still do stuff on the computer for like 10 more seconds.(It sounds like it restarted). Other way is it will just go black screen and the computer wont restart but it will stay on, and the red light on the front pannel will just glow red steadally.. and it wont do anything after that, i have to hold power button to shut it off. And last way is that it will just restart it self.(black screen then reboot).
I dont know what to check realy. But i checked CPU and it stays around 45 - 55 C.
So not overheating. But I know when it might crash.. Randomly the screen willl have a little flicker go across the screen. little faded black line go across. But if u have questions for me i try best to answer. Besides that, this is all that i can provide off top of head. Also if i just leave it at desktop doing nothing it wont crash.

We need your system specs including make and model of your motherboard, make, type, and amount of RAM installed, video card, psu make and rated wattage output, etc.

We need to know your security software.

How old is your system? Have changed any of the hardware or installed any software recently? What are parameters of your cpu for temperature?

Are all your fans working, i.e., case, cpu, video card, etc.? What is the system temps?

Do you get any Blue Screens when it crashes and if so what are the error codes?