my computers video card recently got destroyed and now I have a different one inside which I have downloaded the cards driver and now my screen is back to normal but now it keeps randomly restarting at any givin moment and for some reason even when I play a game like stepmania, it lags for no reason without any solution to fix it

the card I had before was AMD Radeon HD 8570 2G now I have this NVIDIA GeForce GT 640