What can I do to get my Sims 3 working smoothly?

My sims 3 game, which, I just bought a month or so ago isn't working right. I have tried a lot. Uninstalling and reinstalling and it still isn't running the way it should be. If I go into Create-a-Sim I have to be quick, or it will freeze. Or make only one Sim at a time, even then it freezes. I don't have any content that's bought, I've disabled custom content from popping up. I can play for a good amount of time, like hours if I can finally get my Sims into a house, but even then sometimes it runs slower in certain parts than others. I've had to save I created into the bin just so I won't loose them completely and all the work is no where. It will even freeze up when I am playing in live mode sometimes. Is there any suggestions? Sometimes it crashes too, but mainly it freezes up. I have changed my options around, and if I do use cheats I noticed it freezes more frequently so I've cut that out. If anybody has had these issues, did you find out a way to improve them? Or did you do something and it made things worse? Please help! Oh if I got an expansion pack would it make things worse or did it help? I have downloaded the latest patch. My computer is an ASUS, with Windows 7, and it has HDMI. Could graphics be an issue?