Did you patch the game? If you didnt then try uninstalling and reinstalling the game according to the second post of this thread:
It should fix your problems. If it doesn't fix your problems then add "Disable Vertex Buffer Objects=1" (without the quotes) into the configuration settings of the .ini; if that doesn't work then you might have to clean your air fan in case youre using a labtop to run the game which might be overheating your computer, resulting in crashes and lag. Take a vacuum hose and put it to the exhaust normally located on the front or spine or side of your notebook. Suck the dust out of it. Then hold it to the intake and suck the dust out of there too. After that buy a can of compressed air for around $10 and blow air for about five seconds into both of those places. If that doesn't help. Then there's not much you can do at this point, sorry.
Make sure to uninstall your mods before you uninstall the game itself. And make sure to keep your saves!
feel free to reinstall your mods after reinstallation! Good luck!