PC blacking out.

[Killzy]

Alright so basically I've had this pc for a very long time and whenever i play or watch something my pc will black out for a second and fully not function at all on my 240hz monitor but with my 60hz monitor on its own my monitor sometimes blacks out for like 5-10 seconds and continues working again which idk why but this information might be crucial. i try re-plugging from the back of my monitor wont connect and wont find signal nor from the pc. i reboot my pc and it'll either work again or will give me this sign (Picture given) Idk what this means but i went into a reddit forums and apparently it has something to do with my graphics card? he said it was BROKEN. But it isn't because i checked my graphics cards and they seem to be working and functioning correctly. i re-plug the HDMI cable into a different socket and sometimes it works sometimes it doesn't its very gambling based. I cant risk into going into a comp game and then blacking out and having to restart my pc fingers crossed that i dont get this black screen with a few blue dots on the bottom right, My friend said maybe its because i haven't cleaned my water cooling tubes or changed the liquid either which i haven't for the past 3 years which yes ik is bad but it has been functioning really well with no problems until now. Do you guys know what the problem is any suggestions? The video i have also sent demonstrates what happens when it blacks out i cannot continue playing or continue to do anything i have to reset my pc (left is the 240hz and the right is my 60hz). Please can someone fix this issue for me!