PC resets

Hey so I’m having this issue with my computer after I recently rebuilt it. once I turn my pc on for the first time it will seemingly work fine until it try and open anything, then my monitors will loose signal, then regain it but everything will be super slow until ultimately the whole pc...

Hi, like a month ago my pc started to randomly resets while gaming, it resets like 5 times in all this time and it's very random. Somedays i can play without any problem. Like a year ago i changed my gpu (sapphire radeon 7870) because it starts malfuntioning, I bought a Sapphire Radeon rx580...