My homebuilt pc wont start games without glitching out pls help me with troubleshooting

When i start up my homebuilt pc everything is alright untill i start up a game or go into fullscreen on a video, it starts glitching which looks like some pixels changing color back to the original blue red and green and a bit of purple and the game getting lower graphics (it all happens suddenly) and and the monitor (which is connected to my pc by hdmi) disconnects
I've tried troubleshooting but i cant find anything does someone know what to do and if so pls tell me

Ps. I'll send out a video off the problem if i can

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