What you're describing sounds like a video issue. I would recommend that you carefully go through the procedures in this answer in our Support FAQ to see if you can correct the problem. You also mentioned that the game would slow sometimes, which is usually caused by other applications consuming a high percentage of system resources. You might also want to have a look at this answer in our Support FAQ, which deals with a black screen caused by insufficient system resources to load the game. Let us know what happens!