I have recently bought Portal 2 and it worked fine for awhile. It lets me play for about fifteen minutes and then the screen freezes up and stays that way for about 2 minutes After that, It goes back to my Steam home pageand an error message pops up saying, "Portal 2.exe has stopped working. Windows will close the program and notify you if a solution is available." It then closes Portal 2 but after that, if I click play again it lets me play for about fifteen more minutes then shuts off again. I am running Windows 8 so that might be the problem. Also, I think it might have something to do with my graphics card. If anyone knows anything about this, I would appreciate it if they could shed some light on this situation.