I was having the same problem, I then discovered my pc was getting hot, found a tool to control the fans and it's now running perfectly, no lag, no crashing.
*Possible fix* I was having really bad problems as previously mentioned and came across this video, I did everything it said and my game runs perfectly, obviously if you do it then it's at your own risk but it worked for me. https://www.youtube.com/watch?v=5FWAVe1km_g