My Sims Game Keeps Stuffing Up Help?

OK so my Sims 3 game which I have just installed Supernatural on I will be playing and then after I get to a good point like my Sims are doing well and there skills are high it will freeze and I have to get out of the game and go back in and then it will unfreeze and then after a while it will freeze again and just now my Sim just vanished like he still had an icon but it was just green with no picture and when I selected it you could see the plum bob but the sim was invisible and could still do things but you can't see it please help ill give a high rating for the best answer! HELLLP!