Xbox One X Crashing

Hello, This is going to be a long explanation but is required so you get the full story, So i had a Xbox one X ive had since a few weeks after the launch and has been doing fine untill about half a year ago, my niece wanted a xbox to play minecraft on so i gave her my xbox one x, however the issue its having before i gave it away is it would not play any intensive game like Borderlands 3 or Call of Duty without the xbox crashing and throwing a overheat warning, but since she only needed it for minecraft at the time it could easily play Minecraft and with my dad puchasing a series X i could borrow his for the time being, but recently she's wanting to play graphically intensive games like call of duty, but as soon as she starts up the game, Poof, the xbox crashes and gives a overheat warning, a few weeks ago i bought a new series X and now my dads xbox one x has been sitting, so we've decided to give my niece my dads xbox one x as his is working perfectly and easily plays call of duty or borderlands 3 with no issues and i get my broken one back and try to repair it myself

So, ive got my xbox back and i want to see if i can fix it, my guess it could be the heatsink/vapour chamber, ive tried replacing the PSU and changed out the crusty old thermal paster with new paste but with no luck, is there any other reason it could crash when loading a intensive game before i decide to take it apart and replace the heatsink, Yes this console is WAY out of warranty before you decide to get it repaired by microsoft

:)