Screen flickers when playing longer (Fortnite)?

I got a used Asrock z270 Pro 4 a long time ago and currently only have the graphics unit of the cpu (i5 6500) and when I play Fortnite it runs quite normally but when I have played for an hour the screen suddenly pops up for a short time to flicker and for a reason I understand that if I screwed the CPU cooler a little more tightly, the ÜC would not boot at all or if I screwed the CPU cooler even more tightly while the PC was on, there would be a graphic error suspect that due to the problem with the cooler the PC is getting too hot and therefore the screen is flickering

Can anyone help with the problem?

What do you use to cool your processor?
Do you happen to have your case model ready and can you tell me where fans are installed in it?
Have you checked in the device manager and / or online whether your graphics driver is up to date?

For checking the processor temperature on the desktop, I can recommend Core Temp. The program is small and free. The temperature should not exceed 90 ° C.