PC shuts off after playing demanding games for an hour or two

I bought a pre-built pc with decent specs for 1080p gaming (Ryzen 5800X, RTX 2080TI, 64gb's of G.Skill DDR4, water cooling, RM 750 PSU), but from the get go this pc was having issues. At first it was blue-screening due to overheating (apparently the water cooling system wasnt turned on? That's been fixed now), and now its just completely dying when running intensive games for around an hour.

I thought I fixed the issue when increasing the GPU fan speed on MSI Afterburner and decreasing the temp limit and for a day or two the gpu wasnt surpassing 74C, but today it began heating up like before, even with lowered settings. Maybe its a PSU issue? Everytime the computor has shit itself I couldnt turn it back on without needing to unplug and replug the powercord. Also my screens get this odd wavy effect from time to time. Maybe thats correlated somehow? Thoughts?